Yes, mail merge templates in Zoho CRM are built and stored using Zoho Writer, and you can edit or reuse them once created.
Click the template's name to open it directly in Zoho Writer and make your changes there, or download the template, edit it in a word processor like Microsoft Word, and re-upload the revised version.
If you want to use one well-formatted template as the starting point for several similar templates, you can copy it and save the copy under a new name, then adjust the copy without touching your original.
Mail merge, along with Email and Inventory print templates, is available on all paid Zoho CRM editions. Free and Trial accounts only have access to the default print option, without mail merge templates.