how do I change the battery in a 96 Cadillac Sedan Deville

I am not familiar with the 1996 you are asking about but I doubt if GM moved engine components around much from my 1993 configuration so here goes. Locate the battery and disconnect the negative (black) cable. Then to the same with the positive (red) cable. Keep a note of other wires that may also be connected to these terminals and be sure all wires are positioned away from the terminals they were attached to. Now locate the hardware that is holding the battery in place. GM typically uses a hold down clamp at the base attached to the body with one nut. Loosen the nut, remove the clamp and set them aside. Now lift out the old battery. With everything loose, examine the wires for corrosion and clean them off with a wire brush and some batttery terminal cleaner. If they are significantly corroded, replace them before moving forward. If the wires are OK. Put a new battery (same type and post configuration) into the vacant location and secure it with the hold down clamp you just removed. Make certain the battery rating (size, amp rating, terminal locations, etc) is correct for your car when you buy it. If it is an open cell type battery, check the water level in each cell to make sure it is at the "fill" mark. Now connect the wires you took off by doing the positive (red) side first and then connect the negative (black) side. If you want, you can install felt washers onto the terminals to help fight corrosion or spray the terminals with a corrosion blocker. Both are available at your parts store or Walmart. Now start your car. If all is OK, turn it off and check to make sure everything is securely tightend down and you are done. Good luck - just go slow and make sure you do not short any tools between battery terminals - oh, and wear some mechanics gloves.