How do I update my Imac (2011) from 10.6.8 to a newer version?

You should stop using Norton. It's of little use, and can actually be harmful. You don't need virus protection - there aren't any in the wild for Macs. And you certainly don't need any 'cleaning' facilities.

If you really want to upgrade you can do so to Yosemite for free from the Mac App Store, as long as you have an absolute minimum of 2GB RAM - though that will struggle and you should really have at least 4GB.

But you should be cautious about Yosemite. It will probably slow your Mac down considerably - it's RAM-hungry and and runs best on SSDs or Fusion drives. Furthermore, any PPC programs you have (such as AppleWorks) will not run under Yosemite unless you go through the hassle of either installing Snow Leopard on a second partition or an external disk, or using Snow Leopard server under emulation in Parallels.

How do I update my Imac (2011) from 10.6.8 to a newer version?

Read https://www.apple.com/osx/how-to-upgrade/.

Be advised that any programs requiring a PowerPC CPU will not work with any OS subsequent to the one you're using now. By now, any program that does would be considered very outdated since the last PowerPC Mac was discontinued about nine years ago. If you are uncertain, install Yosemite on an external device so that you can evaluate it first.

Uninstall the Norton / Symantec "security" product prior to upgrading OS X. Do not reinstall it.

One or more of the following support documents may be applicable:

Uninstall Norton Internet Security for Mac

Removing Symantec programs for Macintosh by using the RemoveSymantecMacFiles removal utility

RemoveSymantecMacFiles

Uninstall any other garbage as well, a category that includes non-Apple "anti-virus" utilities, "cleaners", "optimizers", "accelerators" etc. Back up your Mac prior to making any changes to it. To learn how to use Time Machine readMac Basics: Time Machine backs up your Mac.