How to remove minor surface rust on frame?

Looks like your vehicle has a separate chassis and is not a monocoque design. The chassis will be made from thicker steel than the rest of the body and the small amount of rust that is visible is nothing to worry about, especially if you live in a dry climate.

It will be fairly difficult to remove all of it if you are thinking about just using some sand paper, since the rust appears to be mainly where the pressed metal is joined together and welded.

If I were removing this rust, I would use a wire brush on an angle grinder to get into the corners well, but even then it will be difficult getting to the top of the chassis.

To make the finish look original is going to be hard work, using rust converter, then building up the layers of paint, but you will find that there will be rust in the seams that will eventually come back through.

You could give the rusty bits a rub down with a hand held wire brush to remove any loose rust then brush on some underseal to protect the whole chassis. This waxoyl underseal is what I would use. It won’t look original, but it will protect the chassis well.

It would also be a good idea to spray some similar protection inside the chassis through some of the holes in the chassis. You need to do this only when you know the chassis is totally dry inside.