I have taken a lot of these power wheels apart, there are nylon gears that when the vehicle is powered forward and cannot move the teeth on the plastic gears will wear down and slip.
Here is an ebay site.To replace the gearbox you will need to take off the axle cap and shaft washer that holds the wheel on.
Slide off the wheel.
I think this power wheel has a silver plastic housing that covers the gearboxes and axle, which means that there will be an amount of screws to take out before you can get to the guts.
Once you get to the wires on the motor you will need to cut and strip them and install the replacement
If you can, I would prefer you would crimp the wires together with a blue or Yellow **** splice, but if you do not have crimper or lugs You can use wire nuts large enough to firmly connect both wires. Then electrical tape the nuts and wires to prevent loosening by vibration. Even though these are 6 volt batteries their current is enough for wires not connected well to overheat, melt and could possibly become a fire hazard.
Ideally the wires from the replacement would be the same color as the original but if not you will want to test to see if while in forward it does go forward. If not you need to reverse the two wires. This is a headache saving measure.
Re-assemble everything making sure that the axle nut or cap goes back on Tightly to prevent the wheel from falling off during use.