Without knowing the exact model of your machine I will give yu thi generic answer for most Brother electronic typewriters.
1. Depress the return key so the carrier is at the LH margin
2. Depress the M Rel (margin release) key or function(this may be a CODE + KEY function if the M REL is imprinted on the cover above the number KEYS.
3. Depress the BACKSPACE key until you are at the position you want the left margin location
4. Depress the Margin Set key or function