Open iTunes. Look at Preferences for iTunes (click on "iTunes" on top bar, then Preferences. Now click on the Advanced tab and at the top will be the location for the iTunes Media Folder. Note that location.
In Finder, go to that location and select the iTunes folder that contains the Media folder. Copy that to wherever you want it to exist. It may take a while if you have a lot of stuff in it.
Once it is copied over, go back to the iTunes Preferences page and change the location to the new place you put it. Test iTunes to see that it works. Assuming it does you can delete the old location Media folder.
NOTE: If you do this, and then iTunes starts when the external is not connected and cannot find that location, it may well recreate the original folder in the original location and change the preferences to that, just like it does when it first starts on a new drive. If that happens, you will need to go back into iTunes Preferences and put back the new location.