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

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.

g~ followed by a "motion" will flip the case of the letters.

gU will upper-case them

gu will lower case them

g~w will flip the case of the letters to the end of the current word.

guG will lower case the letters to the end of the file

gU$ will upper case the letters to the end of the current line.

you can do this in normal mode: vEU (having the cursor at the beginning of the word or pressing b to move it there)

v - go to visual

E - go to end of the word

U - make the visual selection uppercase

Instead of the U you can do u for lowercase or ~ for case flip.

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