As a first time changer, it would be best with 2 people. The lower unit can be heavy and awkward by yourself.
1st key is put the motor in gear before dropping the lower unit. This will be crucial in getting it back together correctly.
2nd is there is a bolt under the trim tab(zinc sacrificial anode). This drove me nutz the first time, and I had a service manual to look at when I changed it.
3rd is make sure the water tube is aligned properly when you put it back together.
Depending on when it was last changed, you may only need to replace the impeller and the o-rings rather than the entire pump. This will save you a few dollars, but be sure to inspect the pump housing for any cracks.
When your alarm went off, was your motor peeing water as usual? Dirt daubers are notorious for getting in and plugging the pee hole as they build a nest.
Last, have some ibuprofen ready for your forearms after you wrestle the lower unit back in and get it bolted
This will help your lower back, as well. Good luck!