Emacs has commands for converting either a single word or any arbitrary range of text to upper case or to lower case.
Convert following word to lower case (downcase-word).
Convert following word to upper case (upcase-word).
Capitalize the following word (capitalize-word).
Convert region to lower case (downcase-region).
Convert region to upper case (upcase-region).
M-l (downcase-word) converts the word after point to
lower case, moving past it. Thus, repeating M-l converts
successive words. M-u (upcase-word) converts to all
capitals instead, while M-c (capitalize-word) puts the
first letter of the word into upper case and the rest into lower case.
All these commands convert several words at once if given an argument.
They are especially convenient for converting a large amount of text
from all upper case to mixed case, because you can move through the
text using M-l, M-u or M-c on each word as
appropriate, occasionally using M-f instead to skip a word.
When given a negative argument, the word case conversion commands apply to the appropriate number of words before point, but do not move point. This is convenient when you have just typed a word in the wrong case: you can give the case conversion command and continue typing.
If a word case conversion command is given in the middle of a word,
it applies only to the part of the word which follows point. (This is
comparable to what M-d (kill-word) does.) With a
negative argument, case conversion applies only to the part of the
word before point.
The other case conversion commands are C-x C-u
(upcase-region) and C-x C-l (downcase-region), which
convert everything between point and mark to the specified case. Point and
mark do not move.
The region case conversion commands upcase-region and
downcase-region are normally disabled. This means that they ask
for confirmation if you try to use them. When you confirm, you may
enable the command, which means it will not ask for confirmation again.
See Disabling Commands.