Changing your eBay username:
https://www.ebay.com/help/account/changing-account-settings/changing-username?id=4198
Changing your password:
https://www.ebay.com/help/account/changing-account-settings/reset-password?id=4197
It may take some time for changes to be made and reflected in your account, so for best results, do not change both username and password at the same time.
