I did my knock sensor from the bottom last fall. It wasn't really that bad, just a weird angle that took a few tries to get it in there right. I ordered one of these from amazon and it was the perfect tool for the job. Also lucked out and got my knock sensor on an amazon warehouse deal under $23.

I reset the code and it came back pretty quickly. I also noticed poorer than usual gas mileage (mid teens). For about $50 total you can order a 1 1/16 combo wrench and the knock sensor both from Amazon and DIY in an hour or two. Jack up the front end and go in from underneath. It's a tight spot, but if you can get your hand up in there you'll be fine. It's been 6-7 months since I replaced it and haven't had any issues.