what does ^A stand for in vim

Answer 1

You need to escape the ^ with a \, i.e. s/\^A/^B/. ^ denotes "start of line" in a regular expression. In the replacement text, escaping is not necessary, but possible.

Answer 2

To get the ^A character, press CTRL-V CTRL-A

If you have configured the "paste" action with CTRL-V (generally on Windows) you can use CTRL-Q instead of it.