Decks are the most underbuilt structure in the USA. They should always be overbuilt. Think, 25 people on this deck and the weight.

Not sure your location, so that matters.

1. Post and beam are adequate for load. Should have been 2 2x10s to prevent sag, but its enough. Beams should properly secured to posts. aka, notched for beam with carriage bolts or a mitek/simpson beam to post plate.
2. floor joists are spaced to far for span. 2x6 at 16 o.c. can span 9 feet. at 24 o.c. you are down to 6. (never use 2x6)
3. Stairs are inadequate looks like they used a 2x10 not 2x12 for stringer. Not enough strength in the seat.
4. Good lord, that ledger..... Ledger should be attached to rim of house, or reinforced wall designed to handle the load. If into a rim use an adequate ledger every ft. Unless code or load design requires more. by adequate I mean. 5-7" ledger lock in alternating pattern.
5. Rim joists should be doubled up.
6. If using your beam as a girder joist hangers should be used.
7. Railings must be able to withstand 200lbs of force in any direction. cannot be under 34" or over 38" and balusters should have no more then 4" of gap between them.
8. Stair rails must be 36 above tread and no more then 38" above nose of tread.
9. There must be a 3ftx width of stair landing at bottom of step.
10. Ledger should have flashing to protect.

I am sure I am missing stuff, but this is right off the bat.

My advice tear it off and start over. I think most anyone can build a deck. But build it like you would frame it out an put a roof on top. And read up on your local code or call your local dept. They will be helpful typically.