Sometimes mistakes are made in setting the case for sentences. There are four general categories of capitalization:
Sentence Case - The first letter of a sentence is capitalized
Lowercase - all words are in lowercase
Uppercase - ALL CAPITALS
Title Case - All Words Are Capitalized (This is, really, "Proper case". Title case would be "All Important Words are Capitalized". Nouns, verbs, adjectives, and adverbs should be uppercase. Common articles, prepositions, and conjunctions should be lowercase.)
You can make changes to selected text by going to Format>Change Case and choosing the correct style. (Including tOGGLE cASE)
You could also use a keyboard shortcut. Select the text and then hold down the SHIFT key and tap the F3 key to toggle through three of the main cases – All Cap, Lowercase, and Title.