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:6,10s/foo/bar/g
Find foo and replace with bar in lines 6 to 10 inclusive

:%s/foo/bar/g
Find each occurrence of 'foo' (in all lines), and replace it with 'bar'.

:s/foo/bar/g
Find each occurrence of 'foo' (in the current line only), and replace it with 'bar'.

:%s/foo/bar/gc
Change each 'foo' to 'bar', but ask for confirmation first.

:%s/\<foo\>/bar/gc
Change only whole words exactly matching 'foo' to 'bar'; ask for confirmation.

:%s/foo/bar/gci
Change each 'foo' (case insensitive due to the i flag) to 'bar'; ask for confirmation.

:%s/foo\c/bar/gc is the same because \c makes the search case insensitive.
This may be wanted after using :set noignorecase to make searches case sensitive (the default).

:%s/foo/bar/gcI
Change each 'foo' (case sensitive due to the I flag) to 'bar'; ask for confirmation.

:%s/foo\C/bar/gc is the same because \C makes the search case sensitive.
This may be wanted after using :set ignorecase to make searches case insensitive.

https://vim.fandom.com/wiki/Search_and_replace