Log out (top right corner)
Try to log in.
Choose "forgot password" (small blue link)
On the page that opens, fill in the email address for the account you want to use and click the Send/OK button
Log in to your gmail account.
There will be a temp link there, and if it isn't, check your spam folder.
Click the link in the email.
On the account page here, click Password
Change your password, you don't need the old one.

If you added this new account to BOINC:
Open BOINC Manager
Projects
Select Seti@Home
Click Remove

Tools
Add project
Select Seti
Select Returning user
Add your gmail address plus the password you just set.

That's it.

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