Method 1:
click on them & press the [delete] key
if you can't see them click on the show/hide paragraph marks icon
(normally to the left of the zoom% dialog if your toolbars are on two lines)
Method 2:
choose edit / replace - click in the "find what" box then click on the
"more" button - choose "special" and then click on Section Break - choose
replace all (if that's what you want to do).
But, having said this - are you sure that you want to do this, section
breaks are normally put into documents for a good reason and deleting all
section breaks might change the formatting of parts of your document.
Cheers
JulieD