I have Windows 10 installed on my laptop and I installed Ubuntu onto a partition.
When I boot, it automatically boots into Ubuntu 15.10, I chose to mount the C: drive to /win/ how do I boot into Windows?
What happened is GRUB (GRand Unifying Bootloader) had, for one reason or another, not included the Windows install pointers (so to speak). So it wasn't showing Windows as an option during the boot process.
The easiest way to fix this is to use the terminal. To open the terminal press CTRL+ALT+T.
When the terminal has opened enter the following command:
sudo update-grub
Then press ENTER on your keyboard. At this point you will be asked for your password. Type your password. Note, while typing your password nothing on the screen will change. You will see no asterisks, the cursor will not move. This is normal behavior. Type your password and press ENTER again.
Follow any on-screen prompts (there should be none) and then reboot your system to ensure the changes have taken effect.
