改天 (gǎi tiān)

another day; some other time; to find another day (for appointment etc

Character breakdown

改 (gǎi) · 7 strokes

Phonosemantic compound. 攵 represents the meaning and 己 represents the sound.
天 (tiān) · 4 strokes

Depicts a person (大) with an emphasized head. Based on the original meaning "head". The meaning later shifted to "top", "high", and "sky".