I closed my Facebook account but I need a message I sent for legal purposes. How can I retrieve that message?
This depends on when you closed your account. If you did so very recently (last few days), contact Facebook directly and ask for their help (see https://en-gb.facebook.com/help/181495968648557/). If it's a little longer than that (a few weeks), you might want to talk to a lawyer about that as Facebook may be reluctant to fish our data from old backup sets on your behalf. If it's longer than that then you're probably out of luck as the data and the backup will probably have been overwritten and no longer exists.
THANKS, I CAN DO WITHOUT IT.