There are 2 helpful links:

A) Incomedia's guide, which also allows changing the template of a project.

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How to share a Template using Export/Import function

>> https://guide.websitex5.com/en/support/solutions/articles/44001452473-so-teilen-sie-ein-template-mit-der-imports-und-exportsfunktion

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B) My article in German with with screenshots. Scoll down to the 3rd post marked in blue, this is where my descriptions of replacing the template in an existing project begin.

>> https://helpcenter.websitex5.com/ru/post/257818 (in German, use google translate)