Can I send it back?

I got a laptop from Saturn today, but I'm not happy with it and I wanted to ask if I could return it tomorrow even if it was used a little

Ask Saturn. You do not have an exchange right.

Bought in the branch, it is goodwill, you could have tested on site. You have to ask. Can only be against another device or voucher.

So I could exchange the current one for another and get money back if it was more expensive than the "newer" one

I can't say that. You are not entitled to an exchange because you don't like it. Try your luck!