Q: Am Running

I am running OS x Yosemite on a five year old 17 inch core duo macbook pro

it came new with Snow Leopard.

I have just purchased an I Mac and wish to pass my macbook on to my granddaughter

After I have completed the erase install process I will be back to OS X Snow Leopard

My question is How do I get back to the latest OS X for my macbook

Many thanks for any help that you give me

Trevor