How i can update from 10.7.5 to 10.8

Is there a reason you want 10.8 instead of a newer OS?

Link to purchase at the end.

Check that your computer is compatible with Mountain Lion/Mavericks/Yosemite/El Capitan.

To check the model number hold down the option/alt key, go to the Apple menu and select System Information.

iMac (Mid 2007 or newer) model number 7,1 or higher

Your Mac needs:

OS X v10.6.8 or OS X Lion already installed

2 GB or more of memory (More is better - 4 GB minimum seems to be the consensus)

8 GB or more of available space

Check to make sure your applications are compatible.

Application Compatibility

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

Mountain Lion

You are welcome. Go to System Information, as described above, and then post your iMac model number and how much RAM you have installed.

You should be able to upgrade to Sierra or High Sierra.

I would also go to the site below to see if you can upgrade your RAM to at least 8 GB. The RAM you have will work, but it would be better to have more.

Other World Computing

You may need to update to El Capitan first. Use the links below. Make sure you check the items I posted above.

How to get macOS High Sierra

Sierra download

You can try to download it at the link below. If it won't let you, then your only options are to upgrade to Sierra/High Sierra or find an older computer where you can download it using your Apple ID.

El Capitan