Hi.

I have a button that updates records from my form to a table. On clicking it pops up a message box with an update confirmation. Please suggest a way to customize the message box.

:)

I don't believe that you can "customize" this message. You can prevent the message from appearing, and you can display your own message if you prefer. To prevent the message from appearing put:
DoCmd.SetWarnings False
before your update code. Make sure to turn them back on after the update call though, or no warnings will ever appear for anything.
DoCmd.SetWarnings True