...
Go to the Vendors module > Contacts tab and open the profile of the contact person in question in the edit mode.
Go to the Main Data tab and uncheck the Active box.
Click the Save button at the bottom of the page. The E-mail field has become active.
Provide the new e-mail address.
Check the Active box.
Click the Save button at the bottom of the page.
Click the Invite to Vendor Portal button at the bottom of the page to reinvite the vendor to the Vendor Portal.
...