HOW CAN I UNINSTALL OX S 10.10.05

OSX is the operating system, you can't uninstall it without erasing your Mac, but you can restore the backup you have from before it was installed.

If the computer shipped with an older OS X than the current version running, you may be able to try

& see if the former installed OS X is still available from the App Store purchases, and learn what is

necessary to re-install it (if possible) while taking time and effort to set up backups for what you have

now just in case the older system idea fails to work out...

A backup, to include Time Machine, usually isn't enough; however when seeking to upgrade in the

first place, a clone of the existing complete original OS X and its applications, for off-line restoration

can be very helpful because that can run the computer from a suitable externally enclosed HDD or

an SSD, with appropriate connection and adequate speed. A slow setup may work OK but not the

same as having the other hard drive in the computer attached to the main system bus or logic board.

Once you've backed-up whatever you may want to save, of which some may be accessible at later

time even with a different OS X version, depending on the software used to create your files, you

may be able to use OS X Recovery and OS X Utilities, which may allow you to erase & install the

older OS X. If the hard drive has adequate unused capacity or free-space, it may be possible to

re-partition that free space to create a place for that older OS X. Or see if you can install the old

OS X on an externally enclosed hard disk drive, as a selectable choice from the installers.

There is more to it than uninstalling OS X 10.10.5. Some people liked Yosemite 10.10.5 and prefer

it to El Capitan; or their computer configuration appeared to appreciate the older one more. A few

issues in upgrade to El Capitan in the Mac Mini (graphics) are one reason to try & revert to older OS.

My Mini still has Mavericks 10.9.5, it shipped with 10.9.0 when purchased direct from Apple last Dec.

However the thought to be able to upgrade RAM and HDDs myself was a reason for choosing a Late

2012 i7 Q.C. server with dual 1-TB HDDs. Didn't know the slower spin HDDs can be a bottleneck. And

I had hoped to get the same spec Mini in an earlier OS X, because Mt Lion worked better w/ 4GB RAM.

Could not determine which build & OS X was included, and Apple unable to tell me prior to purchase.

So you could probably revert by reinstalling the original as-shipped OS X if the computer did not ship

with OS X and apps on DVD media. You did not identify your Mini in detail. If the unit shipped with

Snow Leopard and had backup install-restore DVDs, then you'd return to 10.6.8 using DVD + update.

Hopefully the idea works out for you.

Good luck & happy computing! 🙂