How to replace battery in remote?

You are supposed to recharge the battery, not replace it.

Charge your Siri Remote - Apple Support

Only the old-style Apple Remote (aluminum or white) has a replaceable battery.

How to replace the battery in your Apple Remote - Apple Support

You'll have to take it to an Apple Store or Authorized. Maybe the battery itself won't be replaced, but the entire remote, for a price. You'll have to check that with the shop.

Users are not supposed to do it by themselves. It's kind of a bit complex procedure that requires a few tools. You can find the procedure on the internet, but we can't discuss it here.