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Yosemite is only available for people who had downloaded it previously, otherwise it is no longer available.
Os X 10.11.4 For Macbook Pro 1
You could probably upgrade to macOS Sierra if your Mac qualifies, otherwise you will be able to update to
Mac OS X El Capitan from here, Upgrade to OS X El Capitan - Apple Support
As both of these operating systems are a few upgrades newer than Mountain Lion you will need to prepare yourself.
O S X 10.11.4
Make sure you have enough RAM, 4GBs is minimum. Make sure third party apps you use now are supported under El Capitan or Sierra and have updates available, https://roaringapps.com/apps?platform=osx. Software updates for macs since os x yosemite. Also make sure that any peripherals such as printers, scanners etc are supported under El Capitan and Sierra check with the manufacturers of these items for updated drivers / software etc.
Os X 10.11.4 For Macbook Air
Macbook Pro 11 5
iPhoto and Aperture if you had it are no longer supported by Apple in favour of their new Photos app.