Smartphone how can i replace screen protector without air bubbles like Pro does ?

When installing the screen protector wipe the display with a damp cloth first, that will allow the air to escape when encouraged by gentle pressure.
Work the air out from the centre to the outer.

Peel off or otherwise remove the screen protector, spray the adhesive (sticky side) with glass cleaner, then reapply the protector and use a credit card to roll out the air bubbles.

1) Make sure device screen is ABSOLUTELY clean.
2) Make sure your hands, and workspace are also clean and lint free, then DOUBLE CHECK #1 above.
3) The pro's use a "wetting" solution to settle the protector onto the screen but NOTE ! wetness is generally bad for electronics !, so " a little, is good. More is NOT better!"
4) A small plastic squeegee is usually included with any "quality" protector kit. USE IT GENTLY! move the squeegee from the center toward the edges, pushing excess solution or bubbles as you go. Excess pressure stretches the protector and causes bubbles to form as the protector settles.
5) Once the protector is bubble free, in the correct place, and not hanging over the edges. LEAVE IT ALONE 'TILL IT DRIES AND SETS.

NOTES If your protector is one that uses an adhesive coating to hold it in place, DON'T TOUCH THE ADHESIVE SIDE WITH FINGERS (or anything else for that matter) because that's what makes dimples that trap bubbles. This process is proof of the saying "You only get one chance to do it right the first time!". If after the process is complete, and if you find there are still bubbles, you may try again but the likely chance of removing and replacing the same film without damaging it is very small. Fortunately, protector screens usually aren't too expensive (compared to the price of a damaged phone/tablet). Good luck!

Step 1: Clean your workspace. Be sure to get a flat, hard surface without a lot of fabric, liquid, dust, etc.
Step 2: Lay the phone down and clean it well with the cleaning cloth provided. Do not use any kind of cleaning fluid or water, but try to be sure that all fingerprints are gone. Especially be sure no dust or lint particles are on the screen. Before proceeding be sure that there are not a lot of dust particles gathering near you, vaccum and clean the area up a bit more if need be. You want as sterile an area as possible before continuing.
Step 3: Being careful not to touch the sticky side of the screen protector, peel back the tab where indicated. Hold the screen protector only by the sides and line it up with the phone. If you touch the sticky side of the screen protector by accident do not try to clean it off, you will only make it worse. Line up and let the screen protector fall into place from the top of the phone to the bottom. Some people choose to use a credit card or thin piece of plastic to smooth out bubbles as the screen protector makes contact with their device, however this is usually not recommended by the makers of the screen protectors or the phone. The choice is yours, but bubbles are meant to smooth out over time, so take that into consideration.
Steap 4: Once the screen protector is on the device if it is straight and you can see bubbles but no lint or dust beneath it, you are successful, give it a few days for the bubbles to dissipate. If it is not on straight, you may be lucky enough to be able to lift it up and place it better without fault, however this is risky. If lint or dust has gotten between your phone and the screen protector you have likely failed, there is little recourse from here, other than removing the screen protector, cleaning the device, and trying with a new screen protector altogether.

Ease of Application and Removal. You shouldn't have to be an accomplished brain surgeon to apply a screen protector properly. The best protectors go on easily without leaving air bubbles or requiring constant repositioning. If you do need to adjust your protector, it should move easily so you don't have to throw it away if you don't get it right the first time. If you want to remove the protector altogether, it also shouldn't leave sticky adhesive residue on your screen.

Remove the previous cover, Wipe off the screen well with a soft cotton. Then stick the new scree protector over the scree carefully without allowing any air bubbles inside.

To lift the protector off one edge of the display, place a sticky tape on the edge of the protector and then pull on the sticky tape to lift off the protector.
Lift the screen protector from one edge, peel it back to the last bubble then re-fix the protector in place by slowly apply pressure across the proctector as you push the protector onto the display making sure no bubbles forms between the display and the protector.

Bring it back to where you bought it.

When you put the plastic screen on make sure that the plastic and the screen on the phone are absolutely clean. If there is any dirt or debris on either the plastic screen will not sit directly on the phone and bubble up where there is residue.