Substitute across multiple files in Vim

There are a couple ways to find/replace in Vim. The most vim-like way consists of populating the QuickFix list (:help quickfix) with all the files you intend to search across. You can then iterate through these files, applying any commands you desire.

Populating the QuickFix list

My preferred way to populate the QuickFix list is using :vimgrep (:vim for short). This uses Vim's internal grep engine, which is consistent across systems. A typical command is as follows: :vim /searchpattern/ pathtofiles. See :help :vimgrep for more info. Note that this will populate the QuickFix list with all matches from all files at the specified path. This will be useful later.

Running a find/replace on QuickFix entries

Once the QuickFix list is populated, You can substitute across all files just like a single file. See the following command:

:cfdo %s/searchpattern/replacepattern/ | update

:cfdo runs the following command across all files.¹
| update is necessary to automatically save the file before the search move to the next match.

Footnotes:

:cfdo is generally preferred over :cdo: references/cdo-vs-cfdo.html