Whenever Apple updates the software for the iPhone, I get people asking me whether they should update, and if so, when. Here’s my general rule.
For Major Updates, that is the Full OS, as in X to X+1: wait 4 weeks. And if your phone model is the oldest compatible model, consider not doing the update as the new features may slow down the iPhone or iPad considerably.
For Significant Updates to the current OS, as in X.Y to X.Y+1: wait 2 weeks. This is just in case there is an error in the update that didn’t show up in testing.
For Bug Fixes or patches for the current level, as in X.Y.Z to X.Y.Z+1: wait 2-3 days. When the patch is first released, the downloads can be slower than normal, so waiting a few days helps speed things up when you do the patch.
I suggest that you plug into your power charger and run the update over WiFi. Make sure you have plenty of time to be without your iPhone or iPad. Some updates can take an hour or more depending on the size and significance of the update.