To downgrade the previous user from system admin privileges to a regular user, you need to remove their admin rights, typically by modifying their group membership. If the system uses a group like sudo or wheel for admin rights, you can remove the user from that group using the gpasswd or deluser command (e.g., sudo deluser olduser sudo or sudo gpasswd -d olduser sudo). This will revoke their ability to execute administrative commands. After that, the user will only have regular privileges.