You can't. You should have saved the original installer you downloaded before installing. After installation it automaticallly deletes itself.
You can give this a try:
Unfortunately, the link for getting specific hardware versions has disappeared. So basically:
- Get yourself a spare hard drive freshly erased or with a freshly erased partition of around 8 GBs or more.
- Boot to your Recovery HD. Select Reinstall Lion from the main menu.
- From the beginning screen select your spare partition/drive as the target destination then begin the download.
- At the end of the download you will observe an Install button for you to click. Now this is important - as soon as that gadget appears you must abort the installation process. There are only two reliable ways to do this quickly: a. yank the cable for the drive from the computer; b. pull the power on the computer - shut it down quickly.
This will result in your finding a folder on the drive that contains InstallESD.dmg that you can now use to create a bootable USB flash drive installer based on the special build for your model. Use it until the next public release of Lion appears or you move on to Mountain Lion.
If you do not stop the download process immediately upon its completion you will end up with an installer that always boots to a non-functioning installation progress window instead of the normal main menu.
Make Your Own Mountain/Lion Installer
1. After downloading Mountain/Lion you must first save the Install Mac OS X Mountain/Lion application. After Mountain/Lion downloads DO NOT click on the Install button. Go to your Applications folder and make a copy of the Mountain/Lion installer. Move the copy into your Downloads folder. Now you can click on the Install button. You must do this because the installer deletes itself automatically when it finishes installing.
2. Get a USB flash drive that is at least 8 GBs. Prep this flash drive as follows:
- Open Disk Utility in your Utilities folder.
- After DU loads select your flash drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
- Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the partition scheme to GUID then click on the OK button. Click on the Partition button and wait until the process has completed.
- Select the volume you just created (this is the sub-entry under the drive entry) from the left side list. Click on the Erase tab in the DU main window.
- Set the format type to Mac OS Extended (Journaled.) Click on the Options button, check the button for Zero Data and click on OK to return to the Erase window.
- Click on the Erase button. The format process can take up to an hour depending upon the flash drive size.
3. Locate the saved Mountain/Lion installer in your Downloads folder. CTRL- or RIGHT-click on the installer and select Show Package Contents from the contextual menu. Double-click on the Contents folder to open it. Double-click on the SharedSupport folder. In this folder you will see a disc image named InstallESD.dmg.
4. Plug in your freshly prepared USB flash drive. You are going to clone the content of the InstallESD.dmg disc image to the flash drive as follows:
- Double-click on the InstallESD.dmg file to mount it on your Desktop.
- Open Disk Utility.
- Select the USB flash drive from the left side list.
- Click on the Restore tab in the DU main window.
- Select the USB flash drive volume from the left side list and drag it to the Destination entry field.
- Drag the mounted disc icon from the Desktop into the Source entry field.
- Double-check you got it right, then click on the Restore button.
When the clone is completed you have a fully bootable installer that you can use without having to re-download Mountain/Lion.
Note: The term Mountain/Lion used above means Lion or Mountain Lion.
As an alternative to the above you can try using Lion DiskMaker 2.0 that automates the process.
I'm afraid Kappy's instructions won't work for you, if I read your post correctly. Since your computer did not ship with Mountain Lion, you will be prompted for an Apple ID & Password before the installer will download. If I understand your post correctly, you purchased this computer with Mountain Lion already installed. If true, that means you will need the Apple ID & password of whoever purchased the Mountain Lion upgrade before the installer will download. If this is true, this computer should not have been sold to you with Mountain Lion installed. If should have been completely erased & Snow Leopard reinstalled. You would then purchase Mountain Lion yourself.
You need the original install disks or a retail SL disk to erase this computer, as you won't be able to reinstall Mountain Lion without the Apple ID & password of the person who purchased Mountain Lion.