You can take a Bonus Action to give your familiar a number of Temporary Hit Points equal to twice your Wizard level. When you do, you can also change the familiar's creature type and the form it takes from the list provided in the Find Familiar spell.
Once you use this feature, you can't use it again until you finish a Short or Long Rest. You can also restore your use of it by expending a level 2+ spell slot (no action required).