Once you are logged into the online account setup, select Admin->Hosted Checkout config option per the image to the right.
You will see a screen similar to the bottom of the web page that lists all the configurations you have made. You can have many of them, and all you need to do is enter the Checkout_ID in Theatre Manager merchant setup to use the one you prefer.
Click OK
Enter in an alias like 'Theatre Manager'
.
If you see the Checkout_id that you want to change in the list below, you can Edit by clicking the edit button. Then process to the next pages for more information about the setup.