How do I get my mid 2010 27 iMac updated to ios big sur

You can not.

macOS Big Sur is compatible with these computers:

MacBook introduced in 2015 or later

MacBook Air introduced in 2013 or later

MacBook Pro introduced in late 2013 or later

Mac mini introduced in 2014 or later

iMac introduced in 2014 or later

iMac Pro

Mac Pro introduced in 2013 or later

You can't. Big Sur requires a 2014 or later iMac.