Add +1 in a db field

**I have been trying to update and add + 1 in the database model (sessions) field whenever a button is being clicked. i am trying to be able to add +1 in a database field whenever a user clicks on a button and have that +1 record and time when the button was clicked in a HTML table, Thank You All, Much Appreciate** 

***Views.py***

    @admin_only
    @api_view(['POST'])
    def swimmersUpdate(request, pk):
    	swimmers = Swimmers.objects.get(id=pk)
    	cs = swimmers.sessions
    	swimmers.sessions = cs + 1
    
    	serializer = SubSerializer(instance=cs, data=request.data)
    
    	if serializer.is_valid():
    		serializer.save()
    		return Response(serializer.data, status=status.HTTP_201_CREATED)
    		
    	return Response(data=serializer.errors, status=status.HTTP_400_BAD_REQUEST)

***Serializers.py***

    class SubSerializer(serializers.ModelSerializer):
        class Meta:
            model = Swimmers
            fields ='__all__'

***Urls.py***

    path('apiurls/', views.apiurls, name='apiurls'),
    path('swimmers-list/', views.swimmersList, name='swimmers-list'),
    path('swimmers-details/<int:pk>', views.swimmersDetails, name='swimmers-details'),
    path('swimmers-create/', views.swimmersCreate, name='swimmers-create'),
    path('swimmers-update/<int:pk>', views.swimmersUpdate, name='swimmers-update'),
    path('swimmers-delete/<int:pk>', views.swimmersDelete, name='swimmers-delete'),

***Models.py***

    class Swimmers(models.Model):
    	name = models.CharField(max_length=200, blank=False)
    	lastname = models.CharField(max_length=200, blank=False)
    	idno = models.CharField(max_length=200, blank=False, null=True)
    	sessions = models.IntegerField(blank=False)
    	totalsessions = models.CharField(max_length=200, blank=True )
    	dateofpayment = models.CharField(max_length=200, blank=True)
    	phone = models.CharField(max_length=30, blank=False, null=True)
    	date_from = models.DateField(null=True)
    	date_to = models.DateField(null=True)
    	type_choice = (
    		("basic", "Basic"),
    		("3x1 week", "3x1 Week"),
    		("1 session", "1 Session"),
    		("2x1", "2x1"),
    	)
    	type = models.CharField(max_length=200, blank=False, null=True, 
    		choices=type_choice, default=type_choice)
    	ammount = models.DecimalField(max_digits=6, decimal_places=2, blank=False, null=True)
    	registration = models.DateField(default=timezone.now)
    	keenphone = models.CharField(max_length=30, blank=False, null=True)
    
    	def __str__(self):
    		return self.name