There are a number of different methods for changing the margins in FrameMaker. Here are two:
1) Go to the Format menu and choose "Page Layout->Column Layout".
This will bring up a popup window. In the window, move to the "Column Margins" section, and set the Top, Bottom, Left or Right margins to your preferred size. Then click on "Update Entire Flow" to make your changes take effect for all your pages.
2) Select the text column for which you wish to change the margin (Ctrl-click). You will notice that the select box has small black squares all around it. To move the edge of the column move the mouse over one of these black boxes. The mouse pointer will change to an arrow pointing in the direction of the edge. Click the left button and slide the edge until you have it at the desired side. For accuracy there is a reference line that moves along the top of the ruler.
If you do this to a single body page, it will change the margins on that body page. If you do this to a master page, it will change the margins on all the pages associated with that master page.