CTC DOY Printer setup help - adjusting Z-position

Hi all, I was given a CTC DIY printer for Christmas and am just getting to grips with it. I came without a glass to put on the heater pad so I have bought one.Now I find that when the nozzle moved to the corner it fell of the edge. The printer is very much like this: https://ctcprinter.com/product_detail.php?ProId=37My computer is Kubuntu Linux and the program to drive the printer is Ultimaker Cura 4.4.1. I have seen in Youtube people stting up the nozzle for the Z-axis, but cannot see how to do it with this. I am a total newbie to 3D printing, so any help is appreciated. Peter M