Q: Model Name:     iMac   Model Identifier:     iMac9,1   Processor Name:     Intel Core 2 Duo   Processor Speed:     2.66 GHz   Number Of Processors:     1   Total Number Of Cores:     2   L2 Cache:     6 MB   Memory:     4 GB   Bus Speed:     1.07 GHz   Boot ROM Version:     IM91.008D.B08   SMC

What is it you wish to update? Mac OS X? RAM? Something else?

Update what, exactly?

Gotcha. Here's the path. Be sure you meet the RAM minimum requirements, although I would install as much as you can which is up to 8 GBs.

Upgrade Paths to Snow Leopard, Lion, and/or Mountain Lion

You can upgrade to Mountain Lion directly from Snow Leopard. Mountain Lion can be downloaded from the Mac App Store for $19.99. To access the App Store you must have Snow Leopard 10.6.6 or later installed.

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 Mountain Lion 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.

Upgrading to Mountain Lion

To upgrade to Mountain Lion you must have Snow Leopard 10.6.8 or Lion installed. Purchase and download Mountain Lion from the App Store. Sign in using your Apple ID. Mountain Lion is $19.99 plus tax. The file is quite large, over 4 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 Mountain Lion — System Requirements

       Macs that can be upgraded to OS X Mountain Lion

         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.

     For a complete How-To introduction from Apple see Upgrade to OS X Mountain Lion.