Combining vimgrep and cfdo you can easily

Find all instances of search string in the project

:vimgrep /search_string/gj **/*

Check quickfix list is correct

:copen

Find and replace and save files

:cfdo %s/search_string/replace_string/g | update

Source: https://engineeringfordatascience.com/posts/how_to_search_and_replace_across_multiple_files_in_vim/