I'm looking for the thread where you put your new usernames, and I have found it before, but I can't find it anywhere. Does anyone know where it is? I decided making a thread would be easier than searching the forums forever. Please get me the link.

the thread is pinned in the general discussion subforum

Oh, thanks! How do I put the date of birth thing? It's not working.

I don't understand the question.
If it has anything to do with the choosing a new username function in Settings, I cannot help, unfortunately, as I've never used it

For the date of birth, you must verify the date of birth you used when you made a Sylestia account.
If you don't remember the date of birth you used when you signed up for the site, or you possibly misclicked something when signing up, then you won't be able to change your username until the correct date of birth is entered.

The date of birth isn't manually typed, instead, you have to click each of the three boxes and scroll to select the correct option in each one.
In the first box, you have to scroll through the 12 Months and select/click your birth month, in the second, scroll through until you find the day you would have been born and click it, and in the third, you'd scroll and select the year that you were born/the year you registered as having been born in, on the site.

If you are still unable to change your username and it's due to the wrong date of birth, this is what Krinadon has previously said to do:

Link: https://www.sylestia.com/forums/?thread=95208&page=1#2 Author: Krinadon Time Posted: 12/16/2020 at 5:07 PM From: https://www.sylestia.com/contactus/ How can I change the Date of Birth on my account? If the Date of Birth on your account is set to the wrong date, please send an email to Support@Sylestia.com via your account's registered email address and provide your Date of Birth. We can then manually update it for you. Please note, regardless, you must be at least 13 years of age to own an account. If it is discovered that you are under 13 and are also the account owner, we will have no choice but to terminate your account.

And if it's due to you being unable to use the scroll boxes, I'd suggest posting your issue here to see if it can be resolved:
https://www.sylestia.com/forums/?forum=8

No, I fixed the problem. Thanks for the help tho.