A-Best: The TI-83/84/Plus/SE calculators require four AAA batteries. You can purchase a package of them at most convenience stores. All you have to do is lift the back cover (not the slide case) and replace the batteries. Make sure you place them in the correct orientation.
If you also need to replace the back up battery, then you have to unscrew the small lid above the AAA battery compartment. The one you need can be 303 or SR44SW.