This is a "user interface"/documentation issue.
I'm a Force9 customer
I have 7 email addresses + the catch-all . On the "Manage my Mail" I can set the passwords on all the other accounts . However I can't see how to set the password for the "catch all". All the others have an 'edit' link, the catch-all only has a rename. The help seems to provide no advice [ https://www.plus.net/help/email-guides/how-to-use-plusnet-email/ ]
Fixed! Go to the fix.
The username and password for the default mailbox, often also used for catch all, are the same as those for your account which you use to login to the Member Centre. To change that password you would need to change your Member Centre password. This can't be done in Manage My Mail.
If you do change your account password the broadband connection password for your router will also change. If the router is configured for auto set-up that password will update automatically. Otherwise you will have to login to the router administration pages and manually change the connection password.
Re: setting email password
Glad I was able to help.
Thanks for pointing out the lack of guidance on setting passwords for the default mailbox. I'll ask Plusnet staff to add this to the help documentation review list.