Updating a template can be done with a HTTPS PUT request with the updated template schema here, and you can always recover the list of all the templates on a teamspace using the Ticket Template list API call.
Follow these steps to update a custom ticket:
Step 1: Enter the name of your teamspace
Step 2: Enter the template ID that you wish to update, these can be found using the get ticket template list found here.
Step 3: Make Get request for custom ticket template here and fill in the name of your teamspace and template id.
Step 4: Hit the execute button, if the post is successful, copy the JSON body
Step 5: Paste the body in the ‘PUT’ request in step 2 and make modifications as necessary to the JSON body. Hit the execute button when satisfied. Remember to check for any syntax errors in https://www.jsonlint.com before executing the put request.