You can change your Password, Email, and Nickname from your settings page. Usernames can only be changed by the Moderation Team and requests to do so will typically be rejected (we will allow changes only in extreme situations).
From any page, click on your username/nickname located at the upper right corner. In the dropdown menu click on settings. You can also click on this link while logged into your account: https://wearereformed.com/community/settings