Just did the same thing a few months ago.
The hardest part was getting the old ladder out, after getting the new ladder up into the attic.
How I did it:
1. I screwed some L brackets onto the old ladder, used my Sawzall to cut the nails holding the old ladder in place and lifter it up into the attic.
2. I used four small L brackets (from the old ladder) screwed to the new ladder, so when I manhandled it into the opening, it settled in right at the perfect level, then I simply used some lags to screw it permanently into place.
Piece of cake. :smile: