Brush up on
Definition 1 of 1
Pro Tip 1/3Frequency Matters
Repeatedly practicing a specific topic is implied when brushing up on it. 
I'm brushing up on my Spanish verbs every evening.
Pro Tip 2/3Not Starting Anew
The phrase suggests improving existing knowledge, not learning something completely new. 
I'll brush up on my Italian before our trip, since I've spoken it before.
Pro Tip 3/3Casual Tone
The phrase has a casual, informal feel and may not suit formal or academic writing. 
Let's brush up on Shakespeare's plays over the weekend.