Trying to adjust handlebars and hex bolt will not release for raising the handlebars.Any suggestions??

Yes, all bikes have handlebar height adjustment.

How to Adjust the Handle Bars on a Mongoose Mountain ...

DIY Mountain Bike https://www.diymountainbike.com > adjust-handle-bar...
This is done by loosening the bolt and clamp on the front of the stem. Insert a 6mm Allen wrench into the bolt head and turn until the handle bars roll freely.

How to Raise Handlebars on Mongoose Mountain Bike

mtbrules.com https://mtbrules.com > how-to-raise-handlebars-on-mo...
Oct 2, 2022 - You can adjust the handlebars from front to back by loosening the clamp bolt in the stem's front. Insert a 6 mm Allen wrench into the bolt head.

How to Raise the Handlebars on a Mongoose

SportsRec https://www.sportsrec.com > Fitness > Strength Training
Jun 29, 2011 - Other options for raising Mongoose handlebars are adding a new riser bar, which can give 1 to 5 inches of height for your handlebars. A steer- ...

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Product Manuals

Mongoose https://www.mongoose.com > pages > product-manuals
The Mongoose owner's manual contains important safety, performance and maintenance information. Read the manual before taking your first ride on your new ...

There is a bolt, might be a hex , in the center of the handlebar stem. Loosen it and you should be able to raise the bar an inch or 2.

Looks like there's a push on plastic cover on the handlebar head stem. Gently pry the cover out and there will be a bolt underneath it (most likely an allen key/hex key bolt). Loosen the bolt and wriggle/twist the handlebars while adjusting the height of the bars. There will be a minimum insertion line on the shaft of the head stem - do not adjust the height above that line or there will not be enough of the shaft to lock into the front forks. Make sure that bolt is tightened up very firmly once you've made the adjustment and reinstall the plastic cover.

There is a long bolt at the top of the neck which is used to pull a tapered nut into the bottom of neck.
Loosen this bolt by 3-4 turns and tap it down to release the taper. Secure the wheel from turning while turning the handlebars and the handlebars can then be raised or lowered as required. Straighten the handlebars in relation to the wheel and retighten the neck bolt and you are done.

There is a single central connection to the handlebars This is called the "gooseneck". (Ther handlebars slide through the goosenect and then the gooseneck is tightened to hold the handlebars. The connector to tighten is in the front (so you do not get "injured" if you have a collison). Loosten the connector (it may be allen, or perhaps hex nut.). Raise or adjust the handlebars, then tighten. Make sure it is really tight (do not strip the threads). The handlebar has serrations to lock on to the gooseneck so the handlebar - gooseneck bolt will have to be loosened quite a bit to allow the serrations to "jump" to the next serration.

there is either a bolt or a hex key nut there that you turn counterclockwise to loosen the bolt holding the post to the body, adjust to the height you wish, the return the bolt/nut to the point it stops not over-tightening the bolt.
Any other questions, please ask, happy to help
Rate on assistance please
Joe

Run "how to adjust threadless stem" through a search engine for videos.

Usually there is a hex bolt or some kind of bolt in the handlebar stem.
Loosen the bolt 4-5-6 turns.
Bolt will move upward.
Then hit bolt down with wrench.
Handlebars should be loose.
Raise handlebars to position.
Hold bolt upward with fingers while tightening.

If you just need to raise the height of the stem, all you need to do is loosen the hex nut at the top of the handlebar stem with a box wrench or adjustable wrench. This is the head of the stem bolt. Once loosened enough the stem should slide up and completely out (if necessary) of the stem casing. There is a tightening mechanism consisting of two soft metal pieces that tighten against each other when the stem bolt is tightened. When this is loose you can raise the stem to various heights. If you find you cannot raise it to your desired height you may need to replace the stem and bolt. However, if the bike has been sized properly to fit your leg length, you should have no problem there. Once the height of the stem is satisfactory just tighten the bolt while keeping the stem straight and you'll be ready to go!

Depending on how high they need to be raised you may need to consider an adjustable stem or a stem extender. I would recommend taking your bike to a good shop as they will be able to take a look at how your bike fits you and what your options are .Its not just as simple as loosening the hex bolt in the stem and raising it as most stock stems don't give you much extra safe length to work with.