Well, that's odd--the link worked before. Here it is again--or just do an Internet search engine search on "Dell change of ownership," which will lead you to an appropriate link.
Now, just to be clear: the only thing that the above link serves for is, to change Dell's records as to who owns the tablet. It doesn't change anything on the tablet itself. It's just an administrative matter, which can ease matters if your daughter-in-law needs to speak with Dell about the tablet (for help or whatever), or Dell with her.
It's the second half of the above that is more important, from the user perspective: resetting the tablet back to it's out-of-box set-up, which will wipe out all of your own account information, passwords, files, programs, etc.--it will be as if your daughter-in-law is getting a new tablet picked up at the store. I assume that that is what you want to do. Again, the above link has nothing at all to do with the resetting process.
To reset the tablet, do a search on the tablet (on the Start screen, click on the search icon (the magnifying glass) at the upper right; or, on the Desktop, swipe in from the right side of the screen and click on the search icon) and type in and search on the word "reinstall", and then choose the option, "Remove everything and reinstall Windows". When you get to the next option screen, again choose "Remove everything and reinstall Windows", and then follow the instructions. NOTE, AGAIN, THAT THIS PROCESS WILL ERASE ALL YOUR OWN PERSONAL INFO. ON THE TABLET--SAVE ANYTHING YOU WANT TO KEEP FIRST, TO A THUMBDRIVE, BACKUP DRIVE, ETC.
No worries about the Windows key--it is embedded in the tablet's BIOS and does not get erased with the reinstall (just like the key already was installed on the tablet when you first got the tablet). And as to Office, if it originally came installed by Dell on your tablet, it should, I believe, still be there after the re-install--you just will need to go through the Office activation process again, I believe (be sure to keep the physical copy of your Office key safeguarded, and not stored on your tablet). Or if you had purchased Office separately, you will need to, I believe, reinstall and reactivate Office.
Do note that you don't use your earlier backup copy for anything--Windows reinstalls itself in this process, using the information stored on the tablet already by the Windows system.
It really is, in the end, a simple process--I hope this helps and I haven't made it seem complex! And you are a nice father-in-law!