On my Zeus 2 (7 inch screen) there are several ways, the easiest of which is a zoom control box
( "+" and "-") right in the middle of the bottom of the screen
. I can also turn the control knob and both of these are covered in my manual. But it might be different on your model.
They also have a couple of rather helpful guys on their 800 number, as do most manufacturers, but for some reason, calling the folks who make and service
things seems to have gone out of style, now that we have our non-professional peers to ask on the internet! :-)