There is no .fmp13 file extension. Just as versions 7 through 11 used the .fp7 file extension and the same file format, versions 11 and 12 use the same file extension and format.
Your OS has a "default"' setting for each file type. During installation of FileMaker 13, this default setting should have been automatically updated such that double clicking a file with the .fmp12 file extension now launches FileMaker 13. This is a setting that you can change manually, if needed.
Thank you this answers my question.
FileMaker 12 was set as the default "Open with" for .fmp12
I have change the default "Open with" to FileMaker 13 for .fmp12 in Get Info