If you wish to use a different username, you need to sign up with a new account. As every account requires a unique email address, you may want to delete your existing account so that you can use the same email address.
To delete your account, please go to Settings > Delete Account
Fridae Perks are non-transferrable and non-refundable. If you are a Perks member, your Perks subscription will be forfeited when your account is deleted.