To change part of a word, place the cursor on the word, to the right of the portion to be saved. Type cw, enter the correction, and press Esc.
To replace part of a line, place the cursor to the right of the portion to be saved. Type C, enter the correction, and press Esc. This changes the portion of the line from the current cursor position to the end of the line.
Use this command to replace the character highlighted by the cursor with another character. Position the cursor over the character and type r, followed by just one replacement character. After the substitution, vi automatically returns to command mode (there's no need to press Esc).
Correcting transposed characters takes just two keystrokes in vi. Suppose you find that you've typed "teh" when you meant to enter "the". Make the correction by putting the cursor over the first letter to be moved (in this case, e), and then type xp. The e and h will trade places - and vi will automatically return to command mode.
To break a line without affecting text, move the cursor to a space where you want the line to break and type r (for "replace") followed by Return. Note that if you type r with the cursor on a character and then press Return, that character will be replaced by the Return.