Q: how do i delete iTunes from el capitan on my macbook air

I assume you mean deleting the iTunes folder with our iTunes data, not "iTunes" the application...  If you mean something else, please post back with details about what you copied to the external drive.

Did you copy the complete iTunes folder that is in your user account Music folder?  Did you try launching iTunes from the iTunes library on the external drive, to confirm iTunes is able to access all of its media files from the external drive's iTunes folder?  This document describes how to open iTunes using a different library

Open a different iTunes Library file or create a new one - Apple Support

You Choose the iTunes Library.itl file that is in the external drive's iTunes folder.

If you have verified that iTunes is now opening and accessing its data from the external drive, you can delete the iTunes folder that is in your user account Music folder (using Finder).  It may be safest to first move that iTunes folder to another location, such as the Desktop.  Use iTunes for a while, to confirm it has no problems with using the external drive iTunes folder.  If no issues come up, delete the iTunes folder on the Desktop (drag to Trash), and remember to Empty the Trash.

And be sure to back up that external drive with your iTunes data to another external drive.   You can set up Time Machine to do that backup, along with backing up your internal drive.