Enlarge? Yes.
A lot depends on how bad your original videos are.
"Improve the quality"? Debatable. You can certainly change brightness levels, contrast, etc, but we don't know what would satisfy your expectations or at what point you would say "that wasn't worth the trouble, I still can't tell if that is a 1956 or 1957 Chevrolet".
Experiment if you like on a copy of your videos with whatever free or trial software you can find.
I've used "Free Video Editor" with some success, but I don't know if you can still find it. Pick any random application you can find and keep your expectations low.
I don't know if the dreaded AI has made any inroads on the issue.