I would like to amend the metric value when the system should trigger an alert. What should I do?

If you have selected Basic package the values are prefixed for the given metrics, it is not possible to amend them. In case of Advanced or All You Can Eat packages when you log into https://app.warningo.com you can edit your monitors and rules. You have to select there the specific monitor where you can click on the Edit icon. Here, under Monitor settings you can change the metrics filter fields.