Your Tiller account is authenticated through Google. If you'd like to use Tiller with a different Google account than the one you're currently using you'll need to start over with a new trial using the other Google account.
Follow these steps for migrating your Tiller account to a new Google account.
Log in to the Tiller Console using the gmail address currently associated with Tiller.
Open (and bookmark) any sheets for which you want to migrate the data to the new sheets. These sheets will remain in your Google Drive unless you delete them. Read more about accessing your Tiller spreadsheets via your Google Drive.
Log out of the Tiller Console using the log out button at the bottom of the console.
Log in to https://sheets.tillerhq.com/auth/google using the Google account you'd like to use for Tiller access.
Start a new trial by entering your payment information in the Subscription area.
Reconnect your bank accounts to Tiller.