How do i change standard account back to admin account?

There's no way to change ALL your accounts to a normal user - it won't let you.

If you're not trying to get into an account you're not supposed to, the easiest way is to enable the root admin account - a quick google search will show you how.

There is always an admin account on the computer and if there are no other user accounts there will be just the one admin account. So if you are currently using a standard account then all you have to do is sign into the admin account.
It sounds like this is your parents computer and the admin account has a password that your not supposed to have and you want to create your own admin account.

^ have the admin do it for you