Here's the details ...
To change the drive letter:
1. Right click "My Computer" or "This Computer"
2. Select "Manage"
3. Select "Disk Management" from the list on the left side of the window
4. In the list running down from the top middle of the window, right-click your library disk
5. Select "Change Drive Letter and Paths ..."
6. Click "Change" in the popup window
7. Select your new drive letter from the dropdown list
8. Press OK
9. Press OK to the warning popup (by later telling Kontakt about the drive letter change, it WILL run correctly)
10. Close the computer management window
To tell Kontakt where the libraries now are:
1. Launch Kontakt (I launched the standalone version)
2. Open Kontakt Options (the gear icon at the top of the window)
3. Click the database tab at the left side of the options window - you will see the currently incorrect library paths
4. Click "Remove" for all the existing paths, and then click "Add" to navigate and add in the new paths (now on "L" drive). Repeat for each library
If you have a large number of libraries, and you expect repeating step 4 above will be tedious, you can try the following instead. Press "Reset and Scan". I assume you'll be prompted to navigate to where you want Kontakt to look. If so, choose the L drive (or whatever you chose as a letter). Kontakt should now find all the Kontakt libraries on the disk. If you don't get a prompt to choose where to look (I don't recall ... it's been a while!) the worst thing it'll do is scan every connected disk, which will take longer. I would have suggested this paragraph as the first option if I remembered for sure that it asks you to choose where to look.
Good luck!
Pete