Upgrade Summon is a method of Summon that can summon an Upgrade Monster from your extra deck.
Just like Ritual Spell Cards that can summon Ritual Monsters, Upgrade Summon can summon Upgrade Spells Cards.
To activate this summoning method, you need a monster from the field. Then, you can use an Upgrade Spell to evolve that monster into an Upgrade Monster.
The condition is that Upgrade Monsters must be summoned when they're evolved from their current form and have an Upgrade Spell on it.
Here is a text:
Must be Upgrade Summoned with "(Upgrade Spell)", using "(Monster Name)".