App names / IDs are used for indexing purposes on Bubble's side, so they cannot be changed once the app is created. However, once you're on a custom domain and your DNS is appropriately configured, the app name is no longer visible to your end users.

If you'd like to change the way the app name appears in your Account > Apps page for organizational purposes, you can do so by clicking on the app and selecting the pencil icon next to the app's name. This will only change the appearance of the app's name on this page, however, it will not change the app's name or ID elsewhere. You can think of this as an app "nickname".
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​Please note that app IDs are now automatically generated by Bubble when you create an app, and are based on the app name you choose, but may not match the app name exactly.