You can't change an existing project template right away, although there are two methods to edit your template.
1. Edit the project a template is based on
Edit the original project your template was based on to change the template accordingly.
Select the main project you based your project template on
Make the changes you want
The changes will now be visible in the project template
Don't know the name of the original project? Just use the second method and create a new project template.
Note: the adapted project template will only be used for new projects. The changes to this template won't have an influence on the projects which have already been created with that template.
2. Create a new project template
If you don't remember the name of the original template or you don't want to change the current template, it's a good idea to create a new project template.
Create a new project and select the template you'd like to change
After the project is created, you can make the changes you want.
Navigate to the Project info page.
Click on the three dots next to the project details
Choose Save as template
Want to delete the first template? Navigate to Settings > Projects > Project templates. Click the small bin below "Actions" to delete the project.