I bought a fairly used iPhone and I can't update items on my phone

If you are trying to update apps that were on the iPhone when you got it, that means that those are the previous owner's apps, you are not authorized to use them, and the previous owner did not do what they were supposed to. They should have done this: What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support.

Your phone is too old to update to a lot of Apps. Try updating to the latest iOS it can run under Settings > General >Software Update

Pete