s (substitute) command can be used to replace specified characters, and the
S command can be used to replace specified lines.
For example, the following has the same leading string on several lines:
foo_bar_baz1 = a1 foo_bar_baz2 = a2 foo_bar_baz3 = a3 foo_bar_baz4 = a4 foo_bar_baz5 = a5 foo_bar_baz6 = a6
Put the cursor on the
f at the start of the first line and type the following:
fixed then press Esc. That changes
fixed_baz1. Move the cursor to the start of the next line and press
. to repeat the command.
A single character can be replaced with the
r command. For example, with the cursor on "f", type
rt to replace the "f" with "t". That only works with characters that can be entered with a single key press, and
s can be used to for a replacement with a digraph. For example, put the cursor on the "x" in text "12x106" then press
s followed by Ctrl-K
*X to change the text to "12×106".
If you need to substitute three lines of text, type
3S. Vim deletes the three lines and enters insert mode, waiting for the subsitution text.
Of course, if you're lazy like me and you don't want to count that there are 7 characters that you want to replace, you could use the c command and use a motion to specify how much to kill. For example, in the previous example, I'd type c2t_ to kill foo_bar and to be left in insert mode.
You could select the "foo_bar" characters with Ctrl+V (Visual Block – Ctrl+Q on Windows), press "c" to change the block, make your change to the first line, and press Esc. All the lines you selected will be changed the same way.