There are a couple ways to go about this. The first way is doing it from Debian itself, which can be a bit more involved, and the second way is to reformat your PC as it currently is, install Windows first, and then install Debian on a second partition. The second way is easier, since you won't have to deal with reinstalling GRUB/using EasyBCD to add the Debian install to Windows' boot manager and it usually has less a chance of going wrong.
Since the second way is easiest, I'll start with that way. First, install Windows. You'll want to completely reformat your HDD, including your Debian install, while you do this. Once that's done, shrink the partition using Disk Manager in Windows for your Debian install and leave it unformatted. Then, install Debian to that empty partition. This way is easiest, as GRUB will take over Windows' bootloader, and have Debian and Windows listed.
The first way is as follows:
While in Debian, using Gparted, shrink the partition Debian is installed to, create a new partition, and leave it unformatted. Then install Windows 10 to that unformatted partition however you like (via USB is usually best), and let it format that partition whatever way it likes. Make sure it's that blank partition, otherwise you'll install Windows over Debian and you'll have to install Debian again.
Then you'll have both installs on your HDD, but Windows will have overwritten your GRUB bootloader and won't have Debian listed when you boot up. In this case, you have two options. You can use EasyBCD to create a boot entry in Windows' boot manager so you have options for both Windows and Debian, or you can reboot into a live Linux USB and reinstall GRUB and use that as a bootloader. I personally prefer GRUB myself when I dualboot Linux and Windows, but you can use whichever you like.
If you have UEFI, you don't need to worry about installing Windows first, as both should have their own bootloader and not mess with MBR. Just use GParted to create some free space in your HDD and then install Windows 10 in that free space. By then it should should up in your UEFI boot menu as "Windows Bootloader", if you want to load it from Debian's bootloader (GRUB), run "sudo upgrade-grub" or something along those lines, os-prober should pick Windows's boot loader and add it as an entry in GRUB.
Also protip, Debian has old packages (not that old atm, but still), you can get a newer version of Wine with some patches here.