How to displace a memory address in assembler n times

Very thank you!

Edit:

I get an error if I try to do that in DE

Well, the thing is that I want to do is
- move HL and DE to the first position of the string
- move DE n times (until the last position of the string)
- then, I'll can to swap the characteres of the string

This is my complete code (it works):

CHGET	equ	#009F
CHPUT	equ	#00A2
POSIT	equ	#00C6
INLIN	equ	#00B1


	db #FE
	dw START
	dw END
	dw START
	
	org #8200
		
START:

mostrar_texto_en_pantalla:
	ld		hl,#0505
	call	POSIT	
	ld		hl,pregunta
loop_mostrar:
	ld		a,(hl)
	call	CHPUT
	inc		hl
	cp		0
	jp		nz,loop_mostrar	
	ld		hl,	texto
	ld		de,	texto	
	ld		b,4
;	call	INLIN
	
loop_incrementar_posicion_memoria:
	inc		de
	djnz	loop_incrementar_posicion_memoria
;ahora tenemos DE en la posicion de destino
	ld		b,2
loop_permutar:
	ld		a,(hl)
	ex		af,af'
	ld		a,(de)
	ld		(hl),a
	ex		af,af'
	ld		(de),a
	inc		hl
	dec		de
	djnz	loop_permutar
	ret
	
;constantes
texto:	db	"Hola!"	;5 caracteres
pregunta:	db	"Inserte una frase:",0
	
END:

Then, I need to offset DE (or change the strategy, or following study assembler )

    ld de,texto
    ex de,hl  ; <- this swaps the content of DE and HL (so, now HL = texto)
    ld bc,4
    add hl,bc   ; HL is now "texto + 4"
    ex de,hl  ; <- swaps DE and HL again. So, now DE = texto + 4

Very thank you @santiontanon and Grauw. It works perfectly!