Move the vertices not the faces. Grab a single vert on one side and it's matching vert on the other and move them up. Then just keep doing this with the other pairs to create the arch.

A different way if you want something a little more automatic, is to just delete all the middle extrusions so you just have the two larger cubic pieces, making sure the two inner faces on those two are deleted as well...these would be the faces facing each other. Then simply use the bridge tool to create the in-between faces...you can edit the amount in the channel box after using the tool or its option box before using it. Here's a bridge tool visual I googled which should help if you want to go this approach:

https://1.bp.blogspot.com/_QrZOY8bgjT...0-h/Bridge.jpg

If you want to start from scratch you could also just extrude a cube along a curve and adjust as necessary.