Unetbootin is a software application, the purpose of it is as indicated above to create a bootable Linux Live CD on a flash drive rather than on a CD/DVD. There are different versions you can use on a system with a currently installed Linux and also version to be used on a windows (what you want or have) or a Mac. Since you already have the peppermint iso, you just need to open unetbootin and navigate to the folder/directory where the peppermint iso is. If you open unetbootin, a graphical interface will appear and in the lower right is a tab with nothing on it except three dots: ... Click that tab and a new windows opens. You will need to know where the peppermint iso is so you can find it. The steps are pretty simple and the unetbootin page does have instructions. Go to the site below and scroll down to Installation and Screenshots to see step by step. Ignore the part at the top about Selecting Distribution as you already have it.
http://unetbootin.sourceforge.net/
