How do I update iMac with snow leopard 10.6.8?

You should be able to use the Mac App Store to download/install and upgrade your Mac all the way to Mac OS X, 10.11.6 El Capitan.

Make a backup, first. Preferably two.

See My user link here.

https://discussions.apple.com/docs/DOC-10981

Best of Luck to You!`

https://discussions.apple.com/docs/DOC-250003506

Firstly by finding out which Mac model you have and how far it can go; then reporting those facts here.