Lucy,
You mean a textbox you drew yourself.
Select the box, right click on the edge of the box (four arrowhead) chose
format text box,  click on the textbox tab. Top of the dialogue next to
internal margins change whatever you like.
Or if all that seems to elaborate, click in your textbox, make sure the
ruler is on (view - ruler), use the indent markers left/right to change the
indentation which will look like the internal margins have been changed.