Sources can be merged, and the old one discarded, by the Replace command in the Source Workspace.
From the Source Workspace:
1) Select one that you DON'T want to keep
2) Right click > Manage Sources > Replace
3) Choose the source from the picklist to replace this one.
4) Click ok.
You can also Move a citation from one source to another at the source citation screen. Simply click on the down arrow next to the name of the source and select another source for that citation to be attached to. That procedure, however, will not erase the old source - the old source stays in sources.