Changing dimensions of a rectangle

I made three different components but used the wrong dimension and I need to change the width

Easily fixed if we know what you have done.
Attach your model to save us guessing.

It’s the three top rails above the hole

.fireplace mantel.skp (1.3 MB)

You have a wall of raw geometry, a tv component and two window components.

The edges above the hole are just edges that can be deleted or shortened or lengthened.

What are you trying to do?

First, I thought I made the rectangle a componet, which I will add depth to it later. I’m just putting in the rectangles for now and change later.

I need those to go left and right out to the dotted line. It’s a mantel

You can “shift-select” each part of the lines you need to move and then “move” (move tool) them out to the dashed lines. When you do the move, select a part that will easily align/snap to your dashed lines.
Alternatively, delete what you have done and just draw new lines on those dashed guides.
Alternatively, you can “push pull” those existing rectangles out into the room and them push pull the sides out and snap them to your guides.
If you are using push pull on either side without guides or inference marks, after you use a push pull operation on one side of the mantel you can "double click " the push pull on the other side and it will easily duplicate the same amount of push pull distance, just so you can keep things symetrical. But like all things sketchup there are always many different ways to achieve the same results.

fireplace mantel.skp (1.3 MB)

not sure of your design but here is revised model that you could work on. Also pulled the TV out of the wall.
Note, to help pull an element out of a wall, you can use the arrow keys to keep the pull in one direction, or after having selected what you need to pull, use the move tool on existing geometry to control the path, and you can always (after having selected and invoked the move tool, type the distance you want it moved and it will move only by that amount.