Q: how to reset security questions if no reset button can be found

Here's the instruction If you forgot the answers to your Apple ID security questions - Apple Support

No reset link means that you don't have a verified rescue email address on the account, you will have to contact Support in the country where you are (and which is therefore the country on your account) to get the questions reset : Contact Apple for help with Apple ID account security - Apple Support

Why do you need to reset them now ? You cannot use the Japan store (both buying new items and redownloading past purchases) unless you are physically in Japan

You can't purchase from the Japan store, including in-app purchases, unless you are physically in Japan. You ca either wait until you are back in Japan, or log into your account (after asking iTunes Support to remove any balance from your account) and put your HK billing address and payment method on your account - you can then contact HK support.