You can spend additional mana on Tiered spells to make them stronger for each additional mana spent (up to the Tier that you have unlocked). For example, a character that has Tier 3 spells unlocked can upcast a Tier 1 or 2 spell at Tier 3 by spending 3 mana on them.
Upcasting
Core Rules v2 p. 45
FAQs
1How does Barrier of Wind interact with Defend?
OfficialCore Rules v2 p. 55
When an ally is attacked at Range, if you upcast Barrier of Wind, you can Defend to protect them provided they are within Reach.