How to change letters in a word from upper case to lower case or the other way around (swap case)?

Question

Instead of deleting the word and retyping all the letters once again in the opposite case, I'd like to find some smart way in Vim to solve the problem.

Answer 1

It's

g~iw 

with the cursor on the word.

Key:

• g flag (I couldn't find a good reference for this...)
• ~ toggle case; alternatively use U for to-upper or u for to-lower
• iw selects the Inner Word, i.e. the word that the cursor is on; ip selects the Inner Paragraph

See Michael Jakl's Vim Introduction and Tutorial - concise and has some nice graphical explanations.