Identifying Replacement Parts for My Bathtub
This is one of those projects that you need to turn the water off, if just hot make sure no one turns another faucet on with cold and hot or water will come out once the valve stem is removed.
Remove the trim
There should be a nut remove the nut (it’s called the packing nut)
The valve body may come out with the nut
If the valve body stays turn the stem and remove
Take that assembly to a local plumbing store.
Match up the assembly.
If it has a rubber washer seal seat get one for the new valve and one for the cold they are cheap,,  buy a box of them then when the new faucet gets hard to turn off you change the washer, seat/seal or whatever you want to call it (it looks like a little round volcano when new when worn out it looks like a flat o ring held in by a brass screw.
Install the new valve with a new washer/seal, seat.
Natal the packing nut there is usually a seal here also
The packing is many times a PFTE string or cord it may be white or black impregnated with graphite.
The PFTE is wrapped around the shaft and the packing nut compresses the seal so it won’t leak.
Replace the trim
Turn water on and verify no leaks. And the valve works.