There's no way to directly modify a template. A good process to accomplish this is:
- Create a new project from the existing template.
- Modify the project to include the updates you need.
- Create a new template from the modified project, and include comments in the Description field to note what was changed in this version.
- Delete the old template.
- Delete the source project.
It's only a handful steps, and it allows you to alter the task plan, reset the schedule start date, modify application folders, update workflow notification options, and other attributes of the project.