This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You cannot replace part of a String; Strings are immutable and have no methods allowing you to do that. You can use the replace/replaceAll methods to create a new String with the alterations, but the best thing to do for Strings you wish to alter is to use StringBuilder.
I won't tell you about using reflection because this is the beginners' forum.