You can edit from the MXD, but it is easier to use ArcCatalog to edit especially if your mxd takes forever to load.
In the MXD you have to insert the model as a tool, assuming you saved it for later in a tool box.
Right Click the tool you inserted and select edit.
If there is an easier way to edit from the mxd I am all ears.
George Houck, that advise is incorrect you can edit a model from within either application, they both open up model builder. Suggest you delete your response.
Len Desson you need to provide more information as requested by Jim Cousins. For example if Edit is not greyed out but nothing appears to open, then it may be as simple as it is opening "off screen". This happens if you have 2 monitors or have remote accessed your machine from a machine with a different display resolution. In this case you can typically see an edge or the right hand side of your desktop so try dragging it into place?
But until you provide more information it is difficult to speculate why a model that is seemingly functional won't open for editing.
You could of said how to do it instead of being rude, especially since so many novices are in here to learn. As a work-around the advice worked, but you are right you can edit from the mxd. I figured out how to edit from the .mxd by going outside of Geonet, where people teach instead of scold, and corrected my response. Feel free to instruct us in better ways to perform our tasks, but toss the tone. So, I suggest you delete your delete comment and remember who the audience is, people who need help and novices.
I came here as I just experienced the same issue (chose Edit... on Model and nothing seems to happen, except that ArcMap border turns gray) and it turns out the model opened, minimized, underneath the Windows toolbar at the bottom of my screen. I discovered that after I changed the toolbar property to Auto-Hide. I was then able to move, resize, and edit the model.
I was having the same issue. I tried your suggestion and it worked. Thank you for the support.
I'm currently using 10.7.1 and i was having the same issue. Existing Model would open but not appear on the screen. i had to disconnect my additional monitors. That brought it back up. Really annoying.