Change individual characters by assigning to them. Write s[0] = 'X'; to replace the first character. Bracket notation works for reading and writing. This modifies in place. Uppercase a character with s[i] = toupper(s[i]); or lowercase with tolower().
These functions from <cctype> convert individual characters. Loop to convert whole words. Strings are mutable. When you modify a character, you change the original. If you need the original intact, copy first with string copy = original; before modifying.