Whenever you perform a system update, it is usually advised that you ensure your phone has enough battery and you have a stable enough connection so that the download doesn’t get interrupted. However once the update is downloading, there’s really nothing you do about it which can sometimes prove to be inconvenient.
However in a report from Android Police, Google is making some changes to that and will actually allow users to pause system updates in the future. While it was initially thought to be an Android O feature, Google engineer Elliott Hughes posted on Google+ suggested that it is part of a change in the Play Store that was being tested internally before anyone really noticed, but presumably it should roll out on a wider basis with Android O.
Hughes also clarifies that this feature won’t just be limited to huge system updates, but small files as well. However he states that this might not be so apparent since smaller files tend to download quicker, meaning that users probably won’t notice until it’s already been downloaded in which there would be no more point in pausing it.
Either way it does sound like an immensely useful feature so if you do want the feature then keep an eye out for its availability in the near future.