How do I change my Account Age?

So I made my account 2017, but I set my age to 2016, and currently I'm 12, but idk how to change my birthday to August 21st, 2008.

Go to your Roblox profile page: https://www.roblox.com/my/account#!/info

Similarly, go to the main Roblox page, click the gear icon on the upper left (in web), and then click on settings. Scroll down and from there you can change your Birthday.

Note that if you set your birthday to a date that will make you under 13, Privacy Mode will be enabled for your profile.

Know more about Roblox' Privacy Mode here: https://en.help.roblox.com/hc/en-us/articles/203313120-Your-Account-Age-Chat-Modes-Filtering