Q: how do i upgrade to 10.10 from 10.8.5

do you already own 10.10 either as a downloaded image or in your apple app store account?

if not then you have no legal way of obtaining it any longer through Apple.

Yosemite is no longer available for new downloads.

As JimmyCMPIT says, unless you have downloaded it in the past.  If you have, then you use Applications -> App Store -> Purchases to obtain another copy (you must be logged in using the same Apple ID used to originally obtain Yosemite).

If you never downloaded Yosemite before then you can move to El Capitan, and maybe (depending on the age of your hardware) Sierra

<https://support.apple.com/en-us/HT206886>

Sierra System Requirements

<https://en.wikipedia.org/wiki/MacOS_Sierra>

Sierra App Store link

<https://itunes.apple.com/us/app/macos-sierra/id1127487414?mt=12>

Check to make sure your applications are compatible.

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

macOS Sierra 10.12: Compatible Apps

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.