Mouse

If you have the original Magic Mouse: Change Magic Mouse batteries - Apple Support

The battery cannot be replaced on a Magic Mouse 2; you need to recharge it with the Lighting cable: Charge a Magic Keyboard, Magic Mouse 2 or Magic Trackpad 2 with Mac - Apple Support

You don't indicate which Mouse you own. If it is a Magic Mouse 2 it has a built-in battery, which is not to my knowledge user serviceable.

If a first generation, see: Change Magic Mouse batteries - Apple Support

If a second generation, see: Recharge the built-in battery in your Apple keyboard, mouse, or trackpad - Apple Support

Newer Apple Mice don't have replaceable batteries. You plug in the cord that came with it and re-charge it.