Why username and suffix are merged in the first field of User logon name of Active directory

Hi,
Hope everyone is safe and well.

We have synced the local active directory to azure by azure ad connect.

All usernames in azure active directory became userorganisation.onmicrosoft.com

I could not separate username from UPN suffix, I started to correct manually the issue per user but as we have several users in the active directory is there a way to correct the username of all users?

Can anyone please advise how can I do that.

Best Regards.