Hi Bek,
The short answer is that updating meta descriptions will have zero impact directly on how well these pages rank, however a more compelling meta description combined with a pretty URL and a decent title tag will help boost click-thru-rate (CTR).
CTR as a ranking factor is the latest Google Conspiracy theories amongst SEOs. Google denies they use SERP data however a lot of professional (including I believe Rand Fishkin, founder of Moz) disagree with this assertion based on their experiences...
So basically updating metas could have an indirect benefit for ranking, but mostly there will be zero impact and thus wouldn't be worth prioritizing over high-impact SEO activities.
If you had to update some, I'f urge that you evaluate which pages already rank on the first page and prioritize them for the best chance of a direct impact.
Or again if you have limited time, just focus on the title tags themselves. These will have the biggest impact!
More Info: https://moz.com/learn/seo/title-tag
I hope this sort of helps and puts you on the right path,
Nick