Changing page number field in a header to an offset other than 0, and after the 
document is saved and closed, then reopened, the offset has reverted to 0.

create a new text document
turn on the header (format>page>header>header on)
make the document at least 2 pages long (insert> manual break)
Put a page number in the header (click in header, Insert>fields>page number)
Now the 1st page is 1, the next is 2 ...

left click to the left of the page number field in the header
right click on the page number field
select fields...
edit the offset to number such as -1 or 1
OK the dialog box
now page numbers have changed acordingly

save and close the document
open the saved document

Page numbers start at 1