You cannot edit the original thread title. You can only set a different title that appears in your post, but that does not affect the original thread title. If you need the original thread title changed (and you're the OP) then you should contact a Moderator and ask that it be changed (along with the reason for the change). Sending Sapience a PM would work but may not be very quick since it requires him to get through all of the PM's he gets until he gets to yours. Better is to put in a ticket to the Community Site department. Tickets get seen by everyone there and not just one person. So there is a better chance to have it addressed sooner with a ticket than a PM.
"Everyone's got one. Better to be a smart one than a dumb one." - me
"It's people like you that [I]think[/I] you know everything that give those of us that [I]do[/I] a bad name." - me
"Eat, drink, and be Merry, 'cause Pippin was a fool of a Took."