A disposal swap is not a single task. It is three lines of work: the power, the drain, and the mount. Sort them before you order the unit, because the split decides whether you can do the whole job yourself or only part of it.
Sort the work
Start with the power. Find out whether the old unit plugs into an outlet under the sink or is hard wired to a switch or circuit. A plug-in unit is usually a matter of unplugging and plugging in the new one. A hard-wired unit means you will be working at the junction box. If the wires do not match what the new unit expects, or no box exists, that line belongs to a licensed electrician.
Next, the drain. The disposal outlet connects to the trap, and the dishwasher hose often ties in at a side knockout on the housing. If the trap or the drain arm must change shape, or the dishwasher connection must be rerouted, you are into plumbing work. That is a plumber's line unless you are comfortable rebuilding the trap assembly.
The mount is the part most owners can handle. The mounting ring seats on the sink flange with a gasket, and the locking tabs must engage firmly. A flange that is cracked, corroded, or no longer seals is a reason to stop and check before the new unit goes on.
What you can own
Shut the power at the breaker, not only at the wall switch, and confirm the unit is dead before you touch a wire. Close the water supply under the sink. Disconnect the drain and the dishwasher hose, loosen the mounting ring, and lift the old unit out. Set the new unit, check that it sits level in the ring, and run water to check the seal before you connect power.
Questions to ask a hired helper
Ask whether the quote includes a new flange gasket, a new drain connection, and removal of the old unit. Ask whether they test for leaks after the water runs, and whether they confirm the dishwasher drain still works after the swap. If the electrical work is in the quote, ask who performs it and whether it is their own crew.
Decide before you buy
Write down the split: power, drain, mount. Mark each line as yours or as a hand's. If the job is mostly yours but the wiring is not, book only the wiring. For planning the unit's service life, see the garbage disposal page: Garbage Disposal: typical service life and when to plan.