Create a new account. eBay does not allow users to change country of registration in most instances.
You will need a different email address for the new account.
You could also request to close this account, and after it's permanently closed you can create a new account with this email - use Guest checkout in the meantime.
Or you can change the email on this account to free up your current email for the new account.
Lots of options there to work that out, but the bottom line is you'll need a new account to register an address in a country other than Singapore.
