Strings replacement with Vim

Replace a pattern within a lonely document

1,$ s/duck/fish/g

1 Position of first character in the text that you want to search to replace
$ End of document
s Search
/ chains separators
g Global Replace all ocurrences automatically

Replace a pattern along multiples files

Select Files.
:args directory/*/*.html

Execute the replace
:argdo %s/regexSearched/String Replace/ge | update