Starting in SOLIDWORKS 2016, modifying a dimension has been made easier with the added Instant2D feature. With Instant2D turned on, you can single click on a sketch dimension for easy access to the dimension input box as well as drag the dimension handles.
Dimension Input Box
Instant2D is on by default in SOLIDWORKS 2016. To turn it off, please go to the Sketch tab in the command manager and toggle off Instant2D.