How to write an accelerando / ritardando?

I'm writing something that starts at 80 bpm. There's a track that arpeggiates between two notes . At the bridge I want to solo this track and gradually accelerate the song to 140 bpm, then slow the it back down to 80bpm for at least one break. Not sure if there is a way to do this or not. If there is, I definitely don't know how. Experienced user, what would you do to solve the problem above?