Here is Vim key combination to switch all text to UPPER case:
The information below is the result of my researches in the Internet and of my experiences. It is solely used for my purpose and may not be suitable for others.
gg - Move to the beginning of the file.
V - Visual mode.
G - Select all text.
U - Switch to lower case.
Here is with search and replace function for the entire file:
Lastly, doing so for single letter:
That's a tilde or squiggly.