You can change your password by logging into members.iracing.com from a web browser (members.iracing.com) and go to Account...My Account.
On the My Account Home page, click Change Password
In the first fields, type in your current password.
In the next two fields, type in what you want to change your password to
Then click the blue Change Password button.
You should now see the message: Your Password was updated successfully.
From the BUI (aka Desktop Icon), you can get there by clicking your helmet at the top right and choose Classic Members Website: