To switch the case of one or more characters use these commands: ~

,

gU

or

gu

Examples:

Switch case of character under cursor (in visual-mode: switch case of highlighted text):

~

Switch case of next three characters:

3~

Switch case of current line:

g~~

In visual-mode: make highlighted text uppercase:

U

Make current line uppercase:

gUU

In visual-mode: make highlighted text lowercase:

u

Make current line lowercase:

guu

Make current word uppercase:

gUaw

Make current word lowercase:

guaw