In order to change your Lnk.Bio Username (URL), log in to your account and head over to the Lnks section.
At the top of the page, tap the Edit button, as shown below.
Once the next modal window open, edit your Lnk.Bio username and tap Save
All done!
Remember to immediately change your Lnk.Bio also on your Instagram profile. You can always follow this guide to do so.