How do I perform a replace operation across more than one file?

Dired mode (M-x dired RET, or C-x d) supports the command dired-do-find-regexp-and-replace (Q), which allows users to replace regular expressions in multiple files.

You can use this command to perform search/replace operations on multiple files by following the following steps:

• Assemble a list of files you want to operate on with either find-dired, find-name-dired or find-grep-dired.
• Mark all files in the resulting Dired buffer using t.
• Use Q to start a query-replace-regexp session on the marked files.
• To accept all replacements in each file, hit !.

Another way to do the same thing is to use the “tags” feature of Emacs: it includes the command tags-query-replace which performs a query-replace across all the files mentioned in the TAGS file. See Identifier Search in The GNU Emacs Manual.