How do you change the time signature (like from the start) when you're writing sheet music with Musescore?

See:
https://musescore.org/en/handbook/time-signatures#add-time-signature

The instructions are great if you've already cretaed the score and then decide to change its time signature. But if you really mean from the start, then the answer is, simply choose what you want in the Create New Score wizard. I might guess you are always starting with the default score, but that's not the usual recommended way to start new scores. Instead, use File / New.