You could do it with sed. Needing to change some but not all of a set of similarly-named files in a folder complicates it a bit:
find . -name conf2 -o -name conf3 -o -name conf5 | xargs sed -i 's/settings1=/settings1=value/g'
If you were changing all of the conf files you could just do this:
sed -i 's/settings1=/settings1=value/g' conf*