VI command – r & s

r

One other replacement edit is given by the r command. r replaces a single character with another single character. You do not have to press ESC to return to command mode after making the edit.

s

The S command, as is usually the case with uppercase commands, lets you change whole lines. In contrast to the C command, which changes the rest of the line from the current cursor position, the S command deletes the entire line, no matter where the cursor is. vi puts you in insert mode at the beginning of the line. A preceding count replaces that many lines.

Both s and S put you in insert mode; when you are finished entering new text, press ESC.