INPUT "Enter a string "; Text$
FOR c% = 1 TO LEN(Text$)
    SELECT CASE ASC(MID$(Text$, c%, 1))
        CASE 65 TO 90
            MID$(Text$, c%, 1) = CHR$(65 + ((ASC(MID$(Text$, c%, 1)) - 65 + 13) MOD 26))
        CASE 97 TO 122
            MID$(Text$, c%, 1) = CHR$(97 + ((ASC(MID$(Text$, c%, 1)) - 97 + 13) MOD 26))
    END SELECT
NEXT c%
PRINT "Converted......: "; Text$