Quality score is track in Google AdWords as it shows each keyword in your account and gives a score out of 10. As it is assigned to each keyword that you have set in your google AdWords account and changes based on a variety of factors that change over time by Google. you have to keep updated yourself every time.
Quality score is updated by Google every time as a search result is carried out in which your ad copy is triggered in SERP page however it can take some time to achieve significant changes in keyword quality score. For more details, you can read Search engine land.
Basically, quality score is all about Google’s rating of the relevance and the quality of the keywords and the PPC ads. Mostly, the quality score is depend on the relevancy of your keywords, text ad and the quality of the landing page. Furthermore it is used to determine cost per click and the ad rank in the ad auction process.