Hi basically it’s described in this post at NextCloud
https://help.nextcloud.com/t/cannot-delete-disable-admin-user-account/67141/2
for DietPi the command would be slightly different:
ncc user:delete admin
Attention: This does, what it should, it delete’s the user and all the files for userid “admin” without any questions and confirmations. Ensure you have created another user with admin privileges before.