I would like to get this manual so I can maintain my system. I am no longer monitored.

I don't think I am allowed, Try,
DSC
Email or phone them from the contacts list. They may help.
Likely you need to Disable communications, and Telephone line monitor to clear the trouble light(Failure to communicate).
I believe the locations are 015, and 380.
Anything in Particular you want to do?
Can you get into Programming Mode, *8 CODE should make the 2 led's flash? ## to Exit.

I need to replace one smoke sensor and want to be clear on procedure. I also plan to replace my two key pads.

I can get you through it, Can you get into programming? If not, it will need to be reset to default programming and codes.
Is the Smoke wireless?
Since you have FOB's, it may be.
I will see if I can find a site with the Manual. I have an Account with DSC, but am sure I can't give it out. I can probably give you a site.
Wireless or Hardwired Smoke?

I don't know if I can get to programming. System is hard wired.
Old sensor is 2112/24T replacement is 4wt-b. When the sensor is changed my next job is to replace the keypads

Sorry, I read 2 Keyfobs, you said 2 keypads?
You will have to assign Keypads after installing, you will need to remove AC and Battery first.
Try *85010, if you get 2 red lights flashing you are in, press ## to Exit.

Since the detector is hard wired, you just need to remove power, swap out detector, then reset clock, with *6 CODE(Master Code), for system programming, it is *8 CODE(Installer Code).

I was able to download the manual from a site I found. Hope fully I will be able to get this done. If I do get stuck, I will be talking to my new best friend. Thanks for your input!

That is a Nice Panel, Expandable to 32 zones, wireless contacts and sensors(motions, glass break, smokes, Keyfobs, etc... )
There is an automation board for lighting and relay control as well as remote arming, disarming, I have a door window transmitter in my vehicle and chimes inside my house if the door in my Van is opened.
I have the 32 LED Keypad in the bedroom and I can see what zone by the location of the LED.
I have the Icon Keypad in the Office and can be programmed 4 different chime sounds, My Van is 1 sound, front door is another, Garage, and Shed in the Back is the other 2 chime sounds so you can HEAR where something is happening. Then at the Entry I have the Alpha Keypad, you can give it Custom Names, and has a 128 event log, so you can tell what the last 128 events were(Maid arming and at what time, What zone went into alarm, etc... )
Can help if you have an Idea. Take Care

Here's your User Manual:
ADT Security System Owner?s Manuals | ADT Security Services