..........because you have a PC without an optical drive (a lot of lightweight laptops are dropping them), my answer is that I have an external DVD drive, it cost about £20 some years ago.
Unetbootin isn't the only utility for creating bootable USBs. With some distros you may even find just 'dd if=somefile.iso of=/dev/sdx' works (where sd'x' is your usb stick). It seems to depend on the distro and depend on the bios of your pc.
Probably the Linux Mint forums are your best bet. I imagine it's sure to be in some FAQ.
To avoid doubt, were you replacing someone's Windows, or adding Mint alongside?
[update]
Google 'unetbootin mint 18'
Take the top hit
https://forums.linuxmint.com/viewtopic.php?t=230340
Excellent discussion there, provided you ignore everything deepakdeshp says. Everyone else in that thread is on the ball. Which doesn't surprise me. The Mint forums, like the Ubuntu forums, are filled with wonderfully helpful and knowledgeable people (though occasionally, as in this example, you have to spot the wannabee-helpful plonkers to ignore!). The stackexchange family of websites (e.g. askubuntu.com) are a wonderful resource as well, the up/down voting of answers there makes them especially high quality.