Characters are limited
A TikTok handle can contain letters, numbers, underscores and periods. A period cannot be at the end.
One change every 30 days
TikTok says a username can be changed once every 30 days. Check the exact handle first, because a change cannot be repeated straight away.
The profile link follows the handle
TikTok says a change also changes the profile link, so any old link you shared stops pointing to you.
Taken handles are a separate question
If another account holds the exact handle, read the TikTok claim page for the routes that exist.
Official sources
Platforms change their help pages and forms often. Follow the wording on your own screen and the current Help Center. Atclaim is an independent service and is not affiliated with Meta or any platform named on this site.