From PC World
Put important files in the cloud: If you plan to work while the PC is in the shop, make sure that the files you need for your current projects are in the folder for your Dropbox, OneDrive, or other cloud-based storage account. That way, the files will be easily accessible on another computer. And any changes you make to those files on that other computer will be synced to your real PC automatically when you get it back.
Backup: Next, create an image backup of your hard drive or SSD. That way, if the professionals wipe your hard drive and reinstall Windows from scratch, you can restore all of your data files. And if they really mess things up, you can restore Windows to the way you had it set up before you sent it to them.
But be warned: If they had a good reason for reinstalling, restoring everything from the image backup could restore the problem that caused the necessity for repair in the first place. Check with the repair people to see if they think this is a good idea.
I recommend you use EASEUS Todo Backup Free and an external hard drive for the backup.