how to update only the file field in model using django rest..i use angular as frontend

I want to update only specific fields in my model. This is my models.py

class CheckinQnResult(models.Model):
    client = models.ForeignKey(User, on_delete=models.CASCADE,null=True, blank=True,related_name='client_create')
    appt = models.ForeignKey(Appointment, null=True, blank=True, on_delete=models.SET_NULL, related_name='appt_qn')
    is_first_visit = models.BooleanField(default=False) 
    document = models.FileField(upload_to='documents', null=True, blank=True)
    duration_of_sickness = models.CharField(max_length=100, null=True, blank=True)
    age_noticed = models.CharField(max_length=100, null=True, blank=True)
    first_sought_age = models.CharField(max_length=100, null=True, blank=True)
    med_treated = models.CharField(max_length=1000, null=True, blank=True)

    

I want to update only specific fields in my model. This is my view.py

              
    class DocumentView(ViewSet):
        model = CheckinQnResult
        serializer_class = DocumentAlldataSerializer
        permission_classes = [IsAuthenticated]
        authentication_classes = [BasicAuthentication, TokenAuthentication, JSONWebTokenAuthentication]
        def create(self,request):
            serializer = self.serializer_class(data=request.data)
            if serializer.is_valid():
                ch_qn = serializer.save()
                appt = Appointment.objects.get(id=request.data.get('appt_id'))
                
                appt.save()
                ch_qn.appt = appt
                ch_qn.save()
                
                return Response({'status': 'success', 'message': 'Check in questions submitted successfully'},
                                status=status.HTTP_200_OK)
                
    
                
            else:
                return Response(serializer.errors)
    
        def getdocument(self,request):
          
            queryset =  CheckinQnResult.objects.filter(client=request.user)
            serializer = DocumentAlldataSerializer(queryset,many=True)
            return Response({'status':'success','data':serializer.data},status=status.HTTP_200_OK) 
           )
        
               
    
        def update(self, request, *args, **kwargs):
            partial = kwargs.pop('partial', False)
            instance = CheckinQnResult.objects.get(id=request.data['id'])
            serializer = self.serializer_class(instance, data=request.data, partial=partial)
            serializer.is_valid(raise_exception=True)
            self.perform_update(serializer)
            return Response({'status': 'success', 'data': serializer.data}, status=status.HTTP_200_OK)
        
        
        def delete(self, instance):
            instance = CheckinQnResult.objects.get(id=self.request.GET.get('id'))
            instance.delete()
            return Response({'status': 'success', 'message': 'Document Deleted successfully'}, status=status.HTTP_200_OK)

I want to update only specific fields in my model. This is my serializer.py

    
    class DocumentAlldataSerializer(serializers.ModelSerializer):
        appt = AppointmentSerializer()
        client=UserSerializer()
       
        
       
       
        class Meta:
            model = CheckinQnResult
            fields = '__all__'
            extra_fields = ['appt','client']

Here I only want to update the document without affecting other field. When I add other field also, it works but I do not want to add other.