Go to https://yobit.net/en/investbox/ and see on the right hand side column, you should see "my investments".

Unless they have changed the terms which I don't see any, you are able to close your plans at any time. There should be an option for you to get your plan closed, and all you need to do is click that button and you should get your balance credited. Although, you'll not get the interest if you haven't invested for 24h.

But please don't invest anything into yobit's investbox, it's most likely some kind of ponzi that is run by yobit. They claim that it's for the devs to promote their coins, but I doubt any devs would put their own money up for grabs to promote already successful coins such as BTC. They're offering just an absurd amount of profit, which resembles a ponzi scheme (although not confirmed).