How to switch between 2 os's?

If you properly installed Linux Mint alongside Windows 10 the grub gives you the option to start up Windows 10 or Linux Mint by using your arrows key up and down and then hitting enter if it does not list Windows 10 on the grub that's because you made a mistake and clicked use the whole disk and have erased Windows 10 entirely and replaced it with Linux Mint. A boot manager utility will do you no good whatsoever you can verify if you accidentally did this by booting up Linux and looking on the C drive and see if he actually did erase Windows 10.

In the event that you accidentally did what I think you did you can reinstall Windows 10 using the Windows 10 media reinstallation tool for free because you already had Windows 10 on there you won't have to pay for a license again and if you want to do a dual boot environment contact me next time and I'll give you specific instructions I'll even walk you through it on the phone it's about a five-minute job

The fact that you can only boot for Linux is probably because you installed both OS's on the same partition. When you installed the second operating system chances are you ended up deleting the hard drive partition for your prior OS. When installing your OS's did you assign them on the same disk partition, or separate ones?

First of all, search the web and search the Mint and Ubuntu forums. Chances are if you are having a problem, a dozen others have already encountered it. Yahoo answers rarely provides the technical answers needed.

Anyways, because some of the other people have suggested that you might have accidentally deleted your windows partition, I would start by verifying your Windows installation is still there. First, go to your system's BIOS or UEFI and look at the boot options/order. Check if Windows is listed and make that your primary boot option. If it boots into windows, you know you haven't deleted it. If it doesn't, boot into linux, and use tools to check for the Windows partitions.

You can use gparted as a GUI based tool in Linux to view information about your drive(s) and parition(s). Or, if you want to check things from a command line you can use the command "parted --list" (without quotes), or maybe "lsblk".

If Windows is still there, you should see a fairly large partition with an NTFS file system for Windows and then a swap parition and an ext4 partition for Linux. Depending on your system, you might also find a few smaller partitions for EFI bootloaders or windows restoration/repair. If the main windows partition is there, try to mount it in Linux to and browse it to see if your data is still there. If it is, that is a good sign that you didn't delete your Windows installation.

If both your windows and linux installations are intact, then the issue is most likely related to your boot loader. This is where you should go to Linux Mint forums. Maybe you installed Linux with the old MBR bootloader instead of the UEFI loader and it doesn't find Windows.... I don't know... The people at the Mint forums are more likely to be able to help you as they wrote the setup processes. The Ubuntu forums might help as well because Mint is based on Ubuntu. Just do everyone a favor and search the forums before posting a question that a dozen people have already asked.

This is what I did. TWO Separate Hard Drives. Hard Drive 0 = Windows 10 Hard Drive 1 = Ubuntu (Hard Drive Numbering starts at zero in Disk Manager)
I installed the Operating separately with only one hard drive connected at a time. Then when I was finished I had them both connected. When I turn on the computer I immediately press the F12 Key and a boot menu comes up. I selected which Hard Drive I want to boot from.