Update from Fortran IV to Fortran 77

Question

I have received an consulting assignment where I will update a Fortran IV program to Fortran 77. The problem is that I never coded in Fortran. Can you give me tips on where I can learn Fortran and what is the difference between the Fortran IV and 77?

Edit

The program to be updated is running on a MODCOMP 9250 (See pictures here) The program was developed on the original compiler. The compiler was updated to Fortran 77 around 1990. My clients want to do some modifications to the application and want to take this opportunity to update the code to the latest compiler