It's not possible to just change an existing project template as such but you can just change the project, the template is based on and the template gets automatically changed accordingly.
However, in case you do not want to change the original project, you can also select the template you want to edit when creating a new project. Adjust whatever it is you want to change in this project.
Afterwards, save this project as another (new) template and delete the first one by going to Settings > Projects > Project templates. Then click on the small bin under “Actions”.
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