Willian, there's a note on the popup that reads "Decreases will take effect on Mar 06, 2017".
Also, here is a help article that might help - https://support.upwork.com/hc/en-us/articles/211062418-Set-a-Weekly-Limit-for-Freelancer-Hours
Please, note that decreases of weekly limit take effect at the beginning of the next work week. Increases take effect immediately. You'll see the explanation on the pop-up when you change your freelancer's weekly limit: