Well, I thought you were pointing to the bullet, obviously! 
I see that you have a custom position for your header, so to not disturb that:
- View > Master Pages
- Control + click on the template frame (the big empty one in the middle of the page)
- Format > Customize Layout > Customize Text Frame > Dimensions. Modify the Width, Height, Top and Left Offsets, and click Apply. Repeat for the facing page, if double sided.

- View > Body Pages and the text will reflow.
Now this is much easier, more intuitive and less math if you use Format > Page Layout > Column Layout and change the margins. But when you have a custom size and/or placement of the running heads (which you do), it moves them back to the default position.

Practice on a copy of the file!