Q: reinstall yosimite

Boot into the Recovery volume (boot with the Command + R keys held down) and select the option to reinstall the system.

It would help a lot, if we knew why you wanted or needed to reinstall Yosemite...?

If your computer is currently working and running Yosemite or El Capitan...?

run > http://etrecheck.com/ and post back with the results, so that we can help with a problem.

If you want wipe the data for a fresh start or are selling your Mac...?

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

If you have upgraded to El Capitan and want to go back to Yosemite...?

see > http://www.macworld.co.delete-el-capitan-go-back-to-yosemite-3581872