This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
Yep, you've got to alter your processing times if you are above sea-level.
The question is, what is your elevation? If you live in the US, and have
Web access, check out the URL
Type in your town, and you should see a geographic summary, including your
elevation in feet.
Just one caveat, from Susan Wood :
I just read your post of the URL for finding out geographic data. When you
include this in the FAQ I think it is important that people understand they
must adjust for their specific location. I just checked my town, Woodbury,
Vermont. The data was OK except I live on a hillside which puts me about
1000 feet above the elevation listed in the data. Anyone in a hilly terrain
owes it to themselves and their families to check a topographic map for their
area and confirm the elevation.