What are the cheapest (free if possible) programs I can use to increase the resolution of photos?

Before I begin, let me start my saying I’m a complete newbie when it comes to photos etc.

I recently received a large number of photos from a family member of older family members (great grandparents / grandparents etc.) and my initial plan was to scan these so my family have access to digital copies of them.

It then got me thinking that in the state they’re in, they’ll be pretty low resolution if I just scan them, however all methods I’ve come across to increase the resolution want me to pay an expensive subscription fee.

Does anyone know what cheaper programs are out there that can solve this issue? I noticed Microsoft Photos which is free has this function, however upscaling appears limited to specific laptops (Copilot+ PCs).