I can't update from Lion 10.7.5 to El Capitan
Please activate (click on the desktop) your Finder and press the Keys CMD + Shift + G copy and paste the following path in the Box and hit Enter.

/private/var/folders



Went into the folder RZ (if is not present look for the folder 35). You will see a new folder witch was replaced xxxxxxx-xxxxxxx. In this folder you find the subfolder C and into it com.apple.appstore. 
Remove this to your desktop (please don't delete unless you are sure that you have not additional problems).  Restart your mac and try to download EL Capitan from the Appstore again.


Additional Information:

If you not able to locate the folder on my steps above open the Terminal copy and paste the following command:

sudo find / -name "com.apple.appstore"


You need to Enter you Password for search. Remember that you not see any input if you type you password. But the Terminal recognize your input.
You get a list in the Output window of the Terminal with location path in /private/var/folder .

Note: If you not use bash as standard Shell you need to type

exec bash


first before you use the command above.