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Most of it comes down to prep and having the right tools. Before you start, kill the power at the breaker - don't just flip the switch. Take a photo of how the plumbing connects under the sink so you remember what goes where. You'll need a bucket, adjustable wrench, putty knife, and usually a disposal wrench or old screwdriver. Disconnect the drain lines (water will spill, hence the bucket), then unscrew the mounting ring holding the old unit in place. It usually twists counterclockwise. Pull the disposal down and out. Clean off the old plumber's putty from the sink flange.
The new one goes in reverse. Apply fresh putty around the flange, set the new disposal in place, and tighten the mounting ring from underneath - hand-tight first, then snug it with the wrench. Don't crank it; you just need it secure. Reconnect the drain lines the same way the old one was set up (that photo helps), hand-tighten first, then get them snug. Fill the sink and run water to check for leaks before you call it done. If something leaks, usually tightening the connection a bit more fixes it. Flip the breaker back on and test the unit.
I did this myself last year and the biggest thing nobody tells you is how annoying the mounting bracket is - it's literally designed to make you want to call someone! Take photos before you disconnect anything, and when you're installing the new one, don't tighten that bracket all the way until you've got the drain lines hooked back up, or you'll be cursing yourself trying to reach the connections. Also keep a bucket underneath because there's always water trapped in the P-trap, and frankly that surprised me way more than it should have.
The hardest part is usually the old unit won't come loose from the sink flange - apply penetrating oil around the collar a few hours before you start, then use a disposal wrench or adjust your approach with a flathead screwdriver to break the corrosion. Once that's loose everything else follows, and the new one seats way easier. Having a bucket under there saves you a mess when you disconnect the drain lines.
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