You want to change your OS? What? Blank? Here's a few tips~

First~the OS you downloaded must be in *iso type.
2nd~you should have a CD-R if the iso is less than 700 mb and DVD-R if the iso is more.
3rd~get some iso burning sofwares~recommended~UltraIso~free~
4th~just put in the CD-R/DVD-R and press BURN at the very lower right.
5th~After the burning;s complete,
6th~Put the OS Disc(now that you made it the OS installer) inside your DVD tray
7th~Shut down
8th~Turn On your computer
9th~press F12 to toggle Boot Options~some comps might have different shortcuts~Depends~
10th~select Boot From Hard Disk
11th~Voila~just follow the installments~~~

Hope i helped~please comment to show that what i posted is helping