UPDATE w3t_Users SET U_Totalposts = XXX WHERE U_Username = 'WHOEVER'

where:
XXX = total number of posts
WHOEVER = user's display name

or:

UPDATE w3t_Users SET U_Totalposts = U_Totalposts + YYY WHERE U_Username = 'WHOEVER'

where:
YYY = number to add to total number of posts

Don't forget the WHERE clause, or you'll change everyone's post count. <img border="0" title="" alt="[Wink]" src="images/icons/wink.gif" />