Please supply more information about your computer's processor type and memory types and amounts, and how much free memory you have left on your hard drive. Depending on the hardware and memory you have, as well as how much free memory is still available, and the age of your computer, you may or may not be able to update it. Also, sometimes, you have to do the updates in increments. For example, you might not be able to make the jump from 10.6.8 to El Capitan. Sems odd you didn't do any of the earlier updates along the way.
I have a Mac Mini that has a 2Ghz Intel Core 2 Duo hard drive, and 2.5 GB 667 MHz DDR2 SDRAM memory and it can't be updated to anything past 10.6.8, despite several people saying it can be. If I did update it, I wouldn't be able to run Quicken for Mac 2006 anymore. Seeing as this is the main reason I still use this Mini, I'm stuck with the old OSX but it runs beautifully. However, I'm now considering getting another hard drive and Mini to back up all the Quicken on to, so I have another working copy of it, for when or if this one every dies, which likely it will one day. Then I'll get a new Mini and run everything else on that.