Unscrew the six small screws on the back with a small phillips cross head screwdriver (an eye glass screwdriver works fine), then carefully prise off the back using a flat blade in the two slots at the top of the calculator.
Keep it upside down or the display will fall out!
You'll see the two 1.5 volt alkaline batteries there, GPA-76 type (flat, round) in mine. Ease them out carefully and replace them, positive side up.
Alternative battery codes (manufacturer dependent) L1154, AG13, GA13, AG76, GP76A, 675HR, LR1154, LR44 per battery web sites, maybe others.
Make sure batteries are seated and display is in its correct position, replace back cover and six screws.
Caveat 1 : You can buy the TI-30XA for about $10-$13 at Staples/Amazon, two batteries will set you back around $5-$7!! But they last a couple years even with heavy use. TI-36X and TI-30X IIS are better calculators and have solar cells as well.....but about twice the price at $17-$25.
Caveat 2: I had one of these where the display faded, replaced batteries, did nothing because circuit board or display was bad. So you could replace batteries and waste the time/money.