you're probably looking for a regular expression something like this:

s/(<a href=".+?)\.html(">)/$1.asp$2/gi
[download]

which should do a case-insensitive global replacement of '.html' at the end of a href value throughout your whole file. You may need to make it more flexible by including some '\s+' or '\.*?' in between portions of your tag depending on if your markup is not consistent.