OK, so let's get you to the right editor first. Don't use the library manager for now, we'll go old-school.
In the Eagle control panel, on the left pane navigator, expand the "Libraries" to find the part that's nearly what you want. Now, from the menu, select "File->New->Library". This will open the library editor. Back on the control panel, right-click the nearly-right part and select "Copy to library". The empty library editor will now be showing a copy of that part, probably the schematic symbol. You want to edit the footprint, so switch to the footprint editor with the icon that looks a bit like a SOT23-6 IC.
For the details of how to do your edits, there are plenty of tutorials out there, some very good. I can never remember the URL for @rachaelATWH4's one but hopefully now I've tagged her she'll pop by and point to it.
Once you've done the edits, save your library in your preferred user location - not where Eagle itself lives as that's likely to be replaced on upgrade. On Windows, many people use "Documents\Eagle\lbr".
What switch are you using? I've used the 10-xx quite a lot, but for illuminated ones I've tended to use a slightly larger one (3FTL6x0) which I think wasn't in the standard libraries.
