What change does this assembly language program need to work?

Question

The program is supposed to take a string and return it in lowercase format. For example: "HEllO" would return as "hello."

Here's the code now, but it works in the opposite way (lower is turned upper):

    .586
.MODEL FLAT

INCLUDE io.h            ; header file for input/output

.STACK 4096

.DATA
prompt1 BYTE    "Enter string", 0
string  BYTE    40 DUP (?)
resultLbl BYTE  "The string is", 0

.CODE
_MainProc PROC
                input   prompt1, string, 40      ; read ASCII characters
                lea     ebx, string
        L2:     cmp byte ptr[ebx], 0
                je end1
                cmp byte ptr[ebx], 'a'
                jl L1
                cmp byte ptr[ebx], 'z'
                jg L1
                sub byte ptr[ebx], 20h
        L1:
                inc ebx
                jmp L2
        end1:
                output resultLbl, string


                mov     eax, 0  ; exit with return code 0
                ret
_MainProc ENDP
END                             ; end of source code

Answer 1

Nevermind find out myself. Just change 'a' and 'z' to 'A' and 'Z' and tell the program to add instead of subtract 'byte ptr[ebx], 20h'.