Vim can find and replace strings across files, just like other text editors. It's really (sort of) easy.
First, load all the files you want to change into the Vim buffer with a splatted directory.
Then, execute the substitution.
:argdo %s/old_string/new_string/ge | update
e flag is important; it tells Vim not to issue an error if the search pattern fails. This will prevent
No Match errors from breaking the chain.