How to convert USB to GUID to Master Boot Record Linux mint
 

Can anybody tell me how to change the partition from GUID to MBR
on a USB drive. I've tried the Disk Management utility and can't seen to get it to work.

Simplest (if you don't want any of the data) is to use fdisk or parted from command line to change the label to msdos. The latter can be scripted, but for one usage, not worth the effort.

Could you explain this to me more. I want to make it a bootable usb and the only way I can do this is to have MBR as partiton.

How you are going to make it bootable? In most cases you do not have to worry about what's on the stick because it gets overwritten anyway.

inside the disk utility management program click a button that makes usb bootable

This program probably marks the partition as bootable, it really does not make your USB stick bootable. What is your goal, what you want to boot from this USB stick?