sed, replace in linux http://cdn1 with https://cdn1
If they are in same directory, you can just do this:
sed -i 's|http://cdn1.domain.com|https://cdn1.domain.com|g' *.html
If not, run find:
find . -name "*.html" -exec sed -i 's|http://cdn1.domain.com|https://cdn1.domain.com|g' {} \;