How do i use my 2010 21.5" iMac as a display with a 2014 Mac Mini

Unfortunately, your iMac is not eligible to become an external monitor (unless you mis-stated the screen size). If you did, you can achieve the result you desire (though I have heard it can be rather temperamental) with Target Display Mode. At any rate, you can find instructions for that here - Use your iMac as a display with target display mode - Apple Support

It may be better to simply purchase a new external monitor. You'll get better quality and longevity.

You are correct, which is why I wrote that Target Display Mode will only work for you if your mis-stated the screen size. I'm afraid if your Mac is really a 21.5" (which you have confirmed), then Target Display Mode will not work. (Though if you have the necessary cables on hand, you may wish to try it out anyway).

Sorry, it cannot be done. A Mac is a computer, not a monitor. Good monitors are available at reasonable prices.

You're welcome- sorry it didn't work out.