Shell Command Help Needed

I know many shell commands, but am trying to learn how to edit a file using a shell editor. Which one is the simplest? ed? less? vi?

I've read the man pages for vi, ed, and less and still am not getting any of the editors to write my changes a file. Here's one of the syntaxes I've tried

vi miniserv.users ,s/adm/admin/g

So, what would the shell command be to change "adm" to "admin" within a file called "miniserv.users"?

 

 

 

 

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