ruleslower=Thread[#-> RotateLeft[#, 13]]&[CharacterRange["a", "z"]];
rulesupper=Thread[#-> RotateLeft[#, 13]]&[CharacterRange["A", "Z"]];
rules=Join[ruleslower,rulesupper];
text="Hello World! Are you there!?"
text=StringReplace[text,rules]
text=StringReplace[text,rules]