To delete a SOLIDWORKS Sketch Dimension you typically select the dimension and hit Delete. To edit the value of the dimension you would double-click on the value to open the Modify dialog.
Are you having difficulties performing these steps? See if you have the option Instant2D enabled. This was introduced in SOLIDWORKS 2016. If you are unaware of what this option does, read through this blog post.
If you single click on the dimension value, it provides a quick way to update the value. But this doesn’t allow the dimension to be deleted. Instead of clicking on the value, select one of the leaders or extension lines and then you can delete the dimension.
The other thing that may be a bit more difficult is to open the Modify Dimension dialog, you have to double-click on the value a bit faster. If you’re too slow on the second click it will think it’s a single click and provide the quick update (which is a bit faster anyway).
For those of you who don’t like this new functionality, all you have to do is deselect the Instant2D button in the Sketch tab of the CommandManager.