convert selection to lowercase (or uppercase) in Sublime Text

From the Sublime Text docs for Windows/Linux:

Keypress            Command
Ctrl + K, Ctrl + U  Transform to Uppercase
Ctrl + K, Ctrl + L  Transform to Lowercase

and for Mac:

Keypress    Command
cmd + KU    Transform to Uppercase
cmd + KL    Transform to Lowercase

Also note that <kbd>Ctrl</kbd> + <kbd>Shift</kbd> + <kbd>p</kbd> in Windows (<kbd>⌘</kbd> + <kbd>Shift</kbd> + <kbd>p</kbd> in a Mac) brings up the Command Palette where you can search for these and other commands. It looks like this: