A. First, troubleshoot a bit. You can stick something down the drain (not your fingers) to try and dislodge whatever may be causing the blockage. You can also try using a shop vac to unblock the drain. If these options don’t work, cut the electricity connection to your device before you go inside to try and remove anything that’s stuck. If you still can’t find the culprit, call a professional. Your unit may be damaged.
To be prepared, measure the distance from the outlet to the top of each disposal before you remove the old unit. If the new unit’s outlet is lower, you must also lower the tee that the discharge pipe connects to. Loosen the two nuts that connect the tee to the tailpiece above and the trap below. Try lowering the tee to see if the tailpiece is long enough. If it’s not, you’ll have to replace it with one that’s slightly longer.