how do I download maverick ?

With the amount of information you've provided it's impossible to offer a solution. We can't see your computer so we need to know the details of your problem and setup, i.e.:

• what system version are you running?
• how much free space do you have on your boot drive?
• how much memory do you have in the Mac?
• what type and model of Mac are you running?
• does you Mac meet these requirements: OS X Mavericks: System Requirements

In other words, you have less than 10 GB of available storage space?

Either delete the files you DO NOT need anymore or have a backup of (though deleting them will turn the backup into the only copy of them), or get an external HDD to store your data.

It is not recommended to have that little available storage space for exceptional cases like this or the system needing space for temporary files.

To find out, where you storage capacity is being used, you can use the following free applications:

• DiskInventory X
• OmniDiskSweeper
• JDisk Report
• GrandPerspective

Maybe try a combination of several, if you still can't find the "missing" capacity.