。
I'm leaving one day earlier than planned.
Literal
Plan-than one-day early depart-[polite-ます].
予定より ('than planned') uses the comparative particle より — 'from, more than.' 発つ (たつ) is a formal/literary verb for departing on a trip, distinct from the everyday 出る. 一日早く is adverbial — 'earlier by one day.' Note how Japanese packs 'earlier than planned' into a compact より phrase where English needs a comparison clause.