When you start casting Find Familiar, you can modify it to summon an empowered familiar. If you do so, the spell is modified in the following ways.
Attacks
Your familiar can attack.
Duration
The spell requires Concentration and its duration becomes 10 minutes for that casting. The spell ends if the familiar disappears or is temporarily dismissed.
Evolved Familiar
Your familiar's game statistics are replaced with the Evolved Familiar stat block, but the familiar retains its senses, traits, speeds, alignment, personality, and creature type. The familiar disappears when it drops to 0 Hit Points or when the spell ends.
Spell Slot Level
If you cast the spell using a higher-level spell slot, use the spell slot's level for the spell's level in the stat block. Its effective spell level is 1 if you cast it without a spell slot.