The firmware version currently running on the EOS 90D is noted on page 5 of the Setup Menu in the EOS 90D.
Please see the image below:
The bottom menu item shows the firmware verson.
NOTE:
The firmware version will not be displayed if the camera is not in one of the Creative Zone Modes. Please ensure that the mode dial is set to one of the Creative Zone Modes (P, Tv, Av, or B) before checking the firmware version.