Sadly, I think that there is no way to do it (personally idk), but after you will finish learning all new words, there are going to be an option "focused review". That's only way that I know)
My understanding is old words will show up for review in typical srs style. In the settings you can set it to review first before learning the new words for the day (this is my preference as I like to ensure I do all my reviews even if I don't get around to new stuff for the day). There's a "focused review" option for reviewing earlier than the srs schedule, but I haven't tried it...
Click the cog wheel (the "settings" button) next to the schedule and there will be an option for Focused Review, which will quiz you on words you've already learned (it will ask you to pick between a few different things to review, such as "learned recently" or "anything in the schedule").
As the people said above, I believe the solution is to click the gear icon next to the lesson and select, “focused review”, and from there you can pick what past words you want to review and study.
