There really isn't an easy way.

If you install Windows over any other linux operating system you will lose
the boot loader. Windows takes over, in other words, it will be impossible
to start anything but Windows. There are ways to repartition and keep both
but the linux boot loader will need repair and this is not easy.

It is far better to install linux over Windows, which always allows dual
booting, i. e. you can choose Windows or Linux at startup. But, since you
have passed the point of doing that I'll have to let more expert users here
walk you through the process of adding Windows to Linux.