is there a way to edit the volume of a wav. file after it has been recorded on a Zoom H4n?
On the device itself, I don't know the answer..you certainly have many more options related to volume processing once you move the audio file onto your computer, which you will end up having to do to make any use of it.
For info on connecting the Zoom to the computer, see the manual.
For free audio editing/processing software see "Audacity"
For more complicated audio problems than changing the volume up or down (for example, distortion, noise, clipping, or reverberations), there are other tools you can use which are often plugins marketed for "audio repair" or "restoration." They take training and practice to use!