How do I save my project in capx?

Its problably an idiot question but i dont know how to do it.

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.