It should be simple. Open your listing's calendar. Next to the name of your listing near the upper left, click on the unlabeled button that has 4 horizontal bars (represents "Menu" in a lot of web sites). In the slide-out menu that appears, click on Pricing. Scroll down until you see "Security deposit" under "Additional Pricing Options". Adjust the deposit amount.
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I am having similar trouble. I have eliminated the security deposit from my listing, on my computer. But, the same listing, viewed on an iPad, still shows a deposit is required. I changed it a week ago! So it shouldn't be a matter of time... I don't seem to be able to access my host status on the iPad - at least, I don't see how to to that from the Airbnb App on that iPad.