Ok, you've downloaded a Linux distro image (.iso). Now what do you do?
Your next step would be to create live boot media:
1) Create a live CD/DVD
OR
2) Create a live USB boot drive Wait! You're not done yet. You still have to configure your computer to boot the media by:
a) Edit/change boot device order in BIOS during bootup
Or
b) Select storage device from boot menu during bootup if your computer offers this option

Note1: if you're already using Linux, search your software manager or package manager for
availability of any of the utilities mentioned below.

Note2: if using UNetbootin, the utility (from my experience) prefers a mount point in /tmp
for the USB drive (already mounted).
Open your terminal and type in the commands below before executing UNetbootin:
sudo umount /dev/sdxy
sudo mkdir /tmp/dirname
sudo mount /dev/sdxy /tmp/dirname
/dev/sdxy=USB drive device
dirname=name of mountpoint