How to repair my USB mouse that only half-clicks.

What make and model is the mouse? Different makes may have different service procedures.

We have a lot of older Logitech M310 wired USB laser mice and they nicely unscrew—once you realize the screws are hiding under paper stickers:

These have microswitches under the buttons which are pretty bulletproof unless the mechanical contact with the button is worn. In that case, you can get a perfectly workable wired laser mouse new for around US$10, which is likely less time and effort than looking for the right replacement microswitch.

If the contact is not worn, try using a soft brush to clean the two contact surfaces, one each switch and button. I'm always surprised at how much lint accumulate inside, when with used on a non-fabric mousepad.

I'm assuming that it's an optical mouse and doesn't have a roller ball. You can try disassembling the upper half from the lower half by removing any visible screw(s) on the bottom. If it still seems stuck together, there could be additional screws under the plastic glide pads. If there are no screws to remove, the upper and lower sections of the housing may be snapped together. Trying to separate them could cause breakage, so you have to be prepared for collateral damage. If you successfully get it opened, you could try a can of compressed air to blow out accumulated dust to restore normal clicking. If the quirk is caused by internal metal and/or plastic fatigue, you may have to replace the mouse.