Use SUBSTRING or LEFT and RIGHT to split your string into parts, that you can cast to upper/lower case respectively, and then concatenate back together.
For example, if the last 3 letters of a string should always be lowercase, and the rest uppercase, do this:
SELECT UPPER(LEFT(MyField, LEN(MyField)-3)) + LOWER(RIGHT(MyField, 3)) AS MyField
FROM ...
For example, this will convert "aBcDe12345fGh" to "ABCDE12345fgh"