MacOS – Download El Capitan Outside of App Store

You can upgrade to Mountain Lion from both Lion and Snow Leopard (SL). With SL you will need to be running the latest version 10.6.8 and then you can just buy it through the app store.

check this link for more details: Apple OSX Upgrade

You just need to make sure your machine can handle Mountain Lion which you can see with this app from the app store Mac Tracker - its a free app.

OS' are like normal apps - they can be installed on up to 5 machines.

Good Luck on the upgrade:-)

First, close the app store or itunes, etc. I fixed this by opening a terminal, then getting a super user terminal: sudo su - enter your password now be careful. You are root.

Next we want to kill all the App Store related processes. This doesn't hurt anything. Let's find them using the ps command and then using the grep search utility to get down to what we want.

MacBook-Pro:~ root# ps -ef |grep store |grep -vi google
  501  9284     1   0 21Dec15 ??         0:08.90 /System/Library/PrivateFrameworks/CommerceKit.framework/Versions/A/Resources/storeassetd
  501  9349     1   0 21Dec15 ??         0:00.73 /System/Library/PrivateFrameworks/CommerceKit.framework/Versions/A/Resources/storeinappd
  501  9353     1   0 21Dec15 ??         0:01.91 /System/Library/PrivateFrameworks/CommerceKit.framework/Versions/A/Resources/storedownloadd
  501  9459     1   0 21Dec15 ??         0:00.17 /System/Library/PrivateFrameworks/CommerceKit.framework/Versions/A/Resources/storelegacy

The second column is the process ID we want. We want to use the kill command:

kill 9284 9349 9353 9459

The numbers will be different on your PC. Open up app store and it should work! I have also read other ways - removing some verisign keys from the key chain app, and others, but this is what fixed me.