Good Morning Fhaas153
Looking at a J-10 picture on eBay
, the picture of the rear says '1.5V battery x 2. Looking at this other shot
It looks like there is a fairly clear Slide-to-Open Battery door at the top of the rear panel. Given the battery rating at 1.5 Volts and the door size, I'd suspect it'll take either AA or AAA batteries. You should be able to slide the door and look, and most battery compartments will have an indication of the specific cell.
To be sure I'm covering both the what and the how, slide the door open. Remove old batteries if still there. Clean up contacts with a bit of fine steel wool if they are corroded or covered with old battery fuzz. Put new batteries in - negative end towards the coil springs generally. Replace sliding hatch.
Hoping this covers what you're looking for!