The c (change) operator deletes the affected text and enters insert mode. The cw is one of the most used commands: it changes one word. The change command is not considered complete until you exit insert mode with ⎋ (escape). Back in normal mode, you can repeat the change with the dot command.
The (uppercase) C command deletes from the current location to the end of the line, and then enters insert mode. It is the same as c$.
Use the r (replace) command to change a single character. Vico waits for you to type the new character. In visual mode, the r command changes the whole selection to the same character.
The s (substitute) command replaces one, or, given a count, that many characters and then enters insert mode. The uppercase variant S is line-oriented and replaces one or more lines.