Web Host Access in Plesk
The section Web Hosting Access in the Websites & Domains panel is one you should generally ignore, so we will not be going into special detail over it. However, this section can be used to change the username you'll be using for SSH later. This is discouraged because it can break things that rely on a particular username. There aren't a lot of programs that do this, but it's not usually worth doing. You can also change the password for the system user here, used for SSH.