My mac os x 10.11 need an update but software update is not available.

My mac os x 10.11 needs to update to 10.13 so that I can install apps and whatnot but I have followed the directions on how to update the software but was not able to because there is no available update. Apple and users, what should I do?This is the model of my iMac Model Name: iMac Model Identifier:iMac9,1 Processor Name:Intel Core 2 Duo Hardware UUID: 38F1A340-305D-504F-8C7F-1EDB176A170B