Re-hanging the gutters.  Not my most favorite thing to do, but you might be able to do it.

One of the reasons I am not big on doing this myself is that on some parts of my house my gutters are over 35 feet off the ground.  That is about 32 feet higher than I like to be when I'm hanging off a ladder.

Let's look at your project though.

First off, are the gutters worth saving?  Do they leak?  Are they rusted?  Are they too small?   If you answer 'yes' to any of those questions you should scrap it and start with new gutters, and if you are going to do that you should call a pro.

The next thing you need to know is the condition of the wood behind the gutter.  If the wood is still in good condition your easiest play will be to use a gutter screw (8" gutter screws are available at your local big box stores) to re-attach using the same holes in the gutter and in your house.

However, if your wood is in poor condition, or the holes are - as we say in Conyers - wallered out, then you have issues.  At this point it might be time to call a pro.

Still in the game?  Ok - good!

Your gutters should have a fastener every 3 feet.  Any further and you are going to suffer the sag again soon.

It is possible that you may have to add a few.

If you skip over any holes, either in the gutter or in the wood you will need to caulk the old hole to prevent water from leaking.  Remember - if you caulk an old hole in a gutter, there will be a corresponding hole in the wood that needs attention.  Get them both.

One more note - there is a lot to be said for having new, one piece gutters installed that come with a gutter cover.  While it may be a little more $$$-wise, you will enjoy the look and your house will enjoy being kept dry.