You can change the Assembly Margins by right-clicking an Assembly and choose Change Assembly Margins option from the menu.
Set the values in this dialog:
The first value represents the left and right margin. The second is for top and bottom. You cannot change the individual left, right, top and bottom margins. Experiment with various values.
Click OK to confirm. Again you will be given the choice of applying the setting to one or all steps:
LPub will redraw the page after you choose Yes or No.