Q: How to use window on my iMac??

We would not do it as these forums are all user to user and we cannot speak for Apple. Is this a brand new iMac? And I assume you are referring to MS Windows? You will need to purchase it/a license and, if you make an appointment at the Genius Bar, they may do that if it fits within the allotted appointment time window. You might want to ask to make sure. You need to use Boot Camp.

Here is some info:

Use Windows 10 on your Mac with Boot Camp - Apple Support

FWIW, you are not running iOS on your Mac.

The first thing is you would have to buy Windows. If you want to install Windows 10 you can follow these instructions…

Use Windows 10 on your Mac with Boot Camp - Apple Support

I don't know if an Apple Store would help you or not, You would have to contact your local store and ask.

shucks

There is always more than one way to say the same thing...... or, even better, blame it on Jive.

Options are described at http://www.macwindows.com/winintelmac.html