The sixth icon down on your toolbar on the left is the straighten/perspective correction. For this demonstration we are working with the straighten.
When you chose it, a bar with hollow squares on the end will show up in the middle of your photo.
As you hover over it, you should get a line with four arrows beneath it. When you get it, click and hold now your left mouse button to move the line to the corner of the object that you want to be horizontal.
Now move the cursor over the opposite end until you get the four arrows with a hollow dot the upper right of the arrows. Left click and hold, bringing that corner up or down to the end of your "horizontal object".
Now all you have to do is click on the okay check on the presets bar.
And watch your photo straighten.
But let's say that it isn't to top of the photo, but something within the picture that you want to have define horizontal. Use the same technique to make the change.
In this image I wanted the roof of the house to define horizontal.
I chose my straighten tool, aligning the ends with the upper and lower parts of the roof. As you can see you don't have to go all the way to the end. You just want to be able to define the angle.
Now I have house with a straight instead of lopsided roof.