Chorus

You touch a nonmagical instrument. Until the spell ends, that weapon becomes a magic instrument with a +1 bonus to spell attack rolls and to the saving throw DCs of your bard spells.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the bonuses increases to +2. When you use a spell slot of 6th level or higher, the bonuses increases to +3.