If you plan on ordering an AirTag, definitely read and bookmark this Apple support post.
It talks through opening your AirTag to get at the battery (super useful) and how to reset it. The way I read it, to reset the AirTag, you need to remove, replace, and press on the battery (until it makes a sound) 5 times.
Arcane, but not something you need to do often. But if you ever do need to reset your AirTag, you’ll be glad you have these instructions.