Its problably an idiot question but i dont know how to do it.
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I'm guessing your asking for Construct 3? Sometimes we get people asking questions about Construct 2 on the Construct 3 forums.
.capx is the format for Construct 2 projects. Construct 3 uses .c3p instead. C3 is capable of reading .capx projects for backwards compatibility but it isn't possible to save as a .capx with C3.
There is a guide on saving files with C3 here.