Updating OS and Safari
How do you update to OS X and latest Safari?
iMac, iOS 1.x
How do you update to OS X and latest Safari?
iMac, iOS 1.x
El Capitan System Requirements
https://support.apple.com/kb/SP728?locale=en_US
Check for application compatibility
Ignore the ads.
How to upgrade?
http://www.apple.com/osx/how-to-upgrade/
Backup the Mac before upgrading.
Best
Go to the App Store and download El Capitan after reviewing what dominic posted. Although you only need 2 GB of RAM to install 10.11, the consensus seems to be 4 GB is the minimum required and more is better.
Updating OS and Safari