From an open Cmap:
To manage an individual resource link, click on its resource link group icon (described in the help section Drag in Resources). After a list of resource links is revealed, right-click (control-click for Mac users) the title of a particular resource link to select it.
A menu will appear allowing you to cut, copy, delete, or edit the resource link. Click Copy to copy the link so that you may paste it (see below) to additional concepts. Click Delete to remove a resource link from the group. You can also edit the resource link using Add & Edit Links to Resources... (described in the help section Add & Edit Links to Resources). Our example will show how to transfer a resource link from one concept to another using Cut.
The icon that linked an image resource to the concept "Roots" has been removed because the resource link that was cut was the only one in the group. Select the concept that you want the resource to link to, then select Paste from the Edit menu. Paste is also used to attach resource links that have been copied. (Another way to paste a resource link is to select a concept, then press Ctrl+v)
Now, the icon and the resource link have moved to a different concept.
To manage an entire resource link group, click to select a resource link group's icon, then select from one of the options in the Edit menu.