If you happen to forget the SSH/RDP password of your server, you can do the following based on the installation type of the OS:
1. If you have installed an OS on your server using already available OS templates (in other words, using the Rebuild process), you can go to the control panel of your server and in the Options tab, there is Reset Password function that you can use to reset the password (new password will be sent to your email)
2. If you have installed an OS on your server using custom ISOs, unfortunately, there's no other way other than rebuilding your server and starting with a fresh OS.