Transfer data from iMac to iMac wirelessly

Nah, don't worry about it - it's actually quite easy. Here are some basic instructions with a link from Apple below. Yes, the good old days of install DVDs - unfortunately, they are no more.

Yours being a 2012, it should have had Mountain Lion on it originally. Once you get that installed and updated, then simply go to the app store and download Yosemite - it's free. And, before you start, make absolutely sure you have a backup of any files you need as you are going to erase the drive so you will lose everything on it. As you want to erase and reinstall the original OS, use Internet Recovery:

Boot up Mac and immediately at chime hold down Command + Option + R keys. Hold down until you see a progress bar (can take a while). You must be connected to the internet.

The Mac will connect to Apple's servers (also takes a while) - once there, you will see a Utilities window.

Choose Disk Utility and choose to erase your entire drive by using the Partition tab > one partition > Mac OS Extended (Journaled) and, under Options, choose GUID Partition scheme.

When finished, go back to Utilities window and choose to reinstall Mac OS X.

That will take a while as the full installer is over 5 GB.

When the download is finished, it should automatically turn into an installer window. Let it install.

At that point, Setup Assistant will pop up and ask you if you want to port over any files - your choice. Follow on screen instructions to connect your external hard drive or, if you only have a few files, you can drag them over later.

During the setup phase, you will also be prompted to enter your info, password, etc.

Once done, launch the app store and check on updates. Hint: if there are a lot, do a couple at a time.

Once you got it updated and are at 10.8.5, you can go back and get Yosemite (and start updating all over, LOL).

https://support.apple.com/en-us/HT201314

(see the "which OS version is installed" especially

https://support.apple.com/kb/PH13871?locale=en_US

(same instructions for Yosemite)

Randy, babowa and rk are correct. Apple says that before you sell or give away a Mac you need to cleanse it and reinstall the operating system as it shipped with:

What to do before selling or giving away your Mac - Apple Support

Otherwise you are violating intellectual property rights. All of us hate paying extra for software and things we can get for free, but when we do that someone else is losing out on their rightful income.

None of the apps on the Mac you have can be updated because they are owned by the AppleID that was used to install them. So, your only option to update is to erase and purchase again on your ID.

If you will read my last post, it includes the directions. Starting with Lion (10.7), Apple went to digital download only. All OS versions, apps, etc. are available at the app store (in your Applications folder).

And yes, you will probably lose iLife; however, you can either make do with the new versions of iMovie and Photos which are included in the OS, or, if you want the old iLife with the (better) version of iMoive and iDVD, then they are still available as retail versions (iLife 09 and iLife 11) from online resellers such as ebay and Amazon.

You're welcome!