There are a few things that you can try, one is to refresh your windows, I haven’t try it but it should work, here is a quick link.
The other way is to do a full restore from the hidden partition that your laptops has, this will remove all your data, not a big thing since you want to sell it, and it will restore it as it was when it left the toshiba factory.
After that, you will unfortunately have to do all the updates and everything that is needed to install windows 10, but hopefully everything should work just fine.
I dont know what keys you need to push during boot, or if there is a Toshiba utility to do that, but I am sure that you will be able to find this info on the manufacturer website.
The folder that has the win10 installation should be on your C: drive and look something like [B]$windows.~BT[/B] but in order to see that file you will have to enable select the option to display hidden files. In my case the size of the folder is almost 4GB, not sure if this can be safely deleted before the upgrade.
In an update that I did on a DELL laptop everything went just fine, and the so far zero issues.