Q3: Convert capital Character to small Character (lowercase)

org 100h

.data

dt1 dB  'COMPUTER'

dt2 db 8 dup(0)

.code

mov ax,@data

mov ds,ax

mov si,offset dt1

mov di,offset dt2 

MOV BL,20H

MOV CX,08

label:

MOV AL,[si]

ADD AL,BL

MOV [DI],Al

INC SI

INC DI

LOOP label

hlt

another way :-
-to convert small Character  to capital Character only when find

org 100h

.DATA
DT1 DB 'University Of Technology' 
DT2 DB 24 DUP(0) 
.CODE 
 MOV AX,@DATA
 MOV DS,AX 
 MOV SI,OFFSET DT1 
 MOV DI,OFFSET DT2 
 MOV BX,0000H 
 MOV CX,18H 

 A:MOV AL,[SI+BX] 
 CMP AL,'a' 
 JGE L 
 JMP X 

 L:CMP AL,'z' 
 JLE L1 
 JMP X 

 L1:SUB AL,20H

 X:MOV [DI+BX],AL 
 INC BX 
 LOOP A

HLT