Upgrading from Snow Leopard (10.6.3) to Mountain Lion
Step Zero: consider your backup and if you want to do a two-step upgrade per http://apple.blogoverflow.com/2015/09/back-up-then-step-up-to-el-capitan/

Step One: Download the 10.6.8 combo updater at https://support.apple.com/kb/DL1399

Step Two: Use the App Store to install OS X and be aware you might need to jump 10.7 to 10.8 to 10.9 to ....