Preferences (or possibly “Options” I’ve got a Mac)=>Advanced=>“iTunes Media folder location” Change…=>enter path/choose folder
Then delete everything from iTunes. It will ask you whether you want to keep the files or send to trash. Send to trash (or “Recycle Bin”). Once iTunes is completely empty, drag the folder from your external HD and drop it into iTunes. It should import pretty quickly since the files are already in the correct folder.
BTW needless to say don’t permanently delete anything until the new music folder has finished importing and you have verified that 1) your songs play and 2) iTunes is playing them from the external HD and not the trash (right click then “Get Info” or “Properties”=>check path at bottom of “Summary” tab).
There may be some minor annoyances like losing some album artwork but that can always be added back later.