Q: I need to download Mountain Lion again.

You must do the following:

Upgrading to Snow Leopard

You can purchase Snow Leopard through the Apple Store: Mac OS X 10.6 Snow Leopard - Apple Store (U.S.). The price is $19.99 plus tax. You will be sent physical media by mail after placing your order.

After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store. Access to the App Store enables you to download Mavericks if your computer meets the requirements.

     Snow Leopard General Requirements

       1. Mac computer with an Intel processor

       2. 1GB of memory

       3. 5GB of available disk space

       4. DVD drive for installation

       5. Some features require a compatible Internet service provider;

           fees may apply.

       6. Some features require Apple’s iCloud services; fees and

           terms apply.

You can then upgrade either to Lion or Mavericks:

Upgrading to Lion

If your computer does not meet the requirements to install Mavericks, it may still meet the requirements to install Lion.

You can purchase Lion at the Online Apple Store. The cost is $19.99 (as it was before) plus tax.  It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation.

     Lion System Requirements

       1. Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7,

           or Xeon processor

       2. 2GB of memory

       3. OS X v10.6.6 or later (v10.6.8 recommended)

       4. 7GB of available space

       5. Some features require an Apple ID; terms apply.

Upgrading to Mavericks

You can upgrade to Mavericks from Lion or directly from Snow Leopard. Mavericks can be downloaded from the Mac App Store for FREE.

Upgrading to Mavericks

To upgrade to Mavericks you must have Snow Leopard 10.6.8 or Lion installed. Download Mavericks from the App Store. Sign in using your Apple ID. Mavericks is free. The file is quite large, over 5 GBs, so allow some time to download. It would be preferable to use Ethernet because it is nearly four times faster than wireless.

    OS X Mavericks- System Requirements

      Macs that can be upgraded to OS X Mavericks

         1. iMac (Mid 2007 or newer) - Model Identifier 7,1 or later

         2. MacBook (Late 2008 Aluminum, or Early 2009 or newer) - Model Identifier 5,1 or later

         3. MacBook Pro (Mid/Late 2007 or newer) - Model Identifier 3,1 or later

         4. MacBook Air (Late 2008 or newer) - Model Identifier 2,1 or later

         5. Mac mini (Early 2009 or newer) - Model Identifier 3,1 or later

         6. Mac Pro (Early 2008 or newer) - Model Identifier 3,1 or later

         7. Xserve (Early 2009) - Model Identifier 3,1 or later

To find the model identifier open System Profiler in the Utilities folder. It's displayed in the panel on the right.

     Are my applications compatible?

         See App Compatibility Table - RoaringApps.

Apple's upgrade instructions are here.

To upgrade from OS X 10.5.8, you must first buy Snow Leopard.

Online (US): http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard

Online (UK): http://store.apple.com/uk/product/MC573/mac-os-x-106-snow-leopard

Germany: http://store.apple.com/de/product/MC573/mac-os-x-106-snow-leopard

Elsewhere: call the phone number in the Apple Online Store

Apple's price is $19.99 or £14.00 or 18,00 €. It is not available to download from any legitimate source (institutional / corporate deployments excepted).

Make sure your system meets Snow Leopard's requirements:

Mac OS X 10.6 "Snow Leopard" System Requirements

To install Snow Leopard for the first time, you must have a Mac with:

• An Intel processor
• An internal or external DVD drive, or DVD or CD Sharing
• At least 1 GB of memory (RAM) (additional RAM is recommended)
• A built-in display or a display connected to an Apple-supplied video card supported by your computer
• At least 5 GB of disk space available, or 7 GB of disk space if you install the developer tools

After you install Snow Leopard you can decide if you want to upgrade OS X to the latest version, Mavericks. Mavericks is available in the Mac App Store, which is available to you after upgrading to Snow Leopard.

Make sure your system meets the requirements for Mavericks:

Mac OS X 10.9 "Mavericks" System Requirements

To install Mavericks, you need one of these Macs:

• iMac (Mid 2007 or newer)
• MacBook (Late 2008 Aluminum, or Early 2009 or newer)
• MacBook Pro (Mid/Late 2007 or newer)
• MacBook Air (Late 2008 or newer)
• Mac mini (Early 2009 or newer)
• Mac Pro (Early 2008 or newer)
• Xserve (Early 2009)

Your Mac needs:

• Mac OS X 10.6.8 "Snow Leopard" or later already installed
• 2 GB or more of memory (RAM)
• 8 GB or more of available hard disk space

If your system does not support Mavericks' requirements, you may be able to upgrade to Lion.

To do that you must phone Apple to order a download code. Redeem the download code in the App Store to download Lion.

Make sure your system meets the requirements for Lion:

Mac OS X 10.7 "Lion" System Requirements

• Mac with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon CPU
  • Core Duo CPUs are not sufficient for Lion
• 2GB of memory (RAM)
• Mac OS X 10.6.8 "Snow Leopard" already installed (update any 10.6.x version to 10.6.8)

To determine which Mac you have, what kind of CPU it uses, and how much memory is installed:

 > About This Mac