Search and replace in all files:

find . -type f -exec sed -i 's/icon-/fa fa-/' {} +

Search and replace in all files: (except .svn directories)

find . -path '*/.svn' -prune -o -type f -exec sed -i 's/icon-/fa fa-/g' {} +

Search and replace in a database column:

Replace 'table' and 'column' with your data.

UPDATE `table` SET `column` = REPLACE(`column`,"icon-","fa fa-");