Your Linux password is emailed to you when you provision a new machine. It is different than your Paperspace password, and any Linux VM you create will have a different password.
The email will arrive from firstname.lastname@example.org with the subject line: Your new Paperspace Linux machine is ready.
Here's an example:
Changing Your Linux Password
You may change your Linux password, but please keep in mind that once you change your password it cannot be retrieved if you forget or lose it!
Enter the "passwd" command as follows to change your password.
Changing password for ben (current) UNIX password: Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully