You can Bulk Edit custom attributes for multiple accounts at once by clicking on the Bulk Edit link in the bottom left corner of the custom attribute screen:
After clicking on that link, there are two main ways of bulk editing, shown below.
Change one attribute at a time for multiple accounts
As shown in the recording below, you can do this by first selecting the accounts that share a certain attribute, and then changing that attribute for all of them at once:
Copy custom attributes from one account to (an)other account(s)
If you have multiple accounts that (largely) share the exact same custom attributes, our recommended approach is to fully fill them in for one of those accounts, save them and then copy them to the other ones.
As shown in the recording below, where we'll copy the custom attributes of the 'Aviato' account to 'Bachmanity' and 'Hooli':