Are you sure you need to edit the template? Do you see an error in the 3cxSystemService.log file related to the changes you did to the Leads entity in Zoho? If so, you will need to take the template from the 3CX server (you can use the "Show template" button for this), copy the contents into a text file, make your changes, and upload it as a custom template. Please note that this will cause that any fix, improvement, new functionality added to the standard template, will not be applied to your installation, so this is not recommended unless there is no other option.