To deauthorize from the current computer, go to the Menu (the Hamburger button) at the top-right corner, and then click "Deauthorize" from the drop-down menu to deauthorize that computer.
If the computer running the DVDFab program is no longer accessible, then log into the DVDFab Member Center, under the License tab, and then deauthorize that computer correspondingly.
NOTE: deauthorizing all computers option can only be done once a year.