You can either click on the Preview Check Button on the toolbar
between print and search:

Or, go to the Flile menu, select Preview, then Check and you should see the screen below with a preview of your check below. Next, click on the Page Setup option on the tool bar, right next to the printer icon.

The Page Setup screen should pop up.
Below you need to adjust the Top margin, which has a default setting (too low) of 0.5".

Change the default to 0" and select "OK" - This will not set the margin to 0", but will force the program to choose the minimum top margin available for your printer driver. (Usually 0.25"-0.33")

This will fix your margin.