Click My PF (left upper corner), click Edit Your Details (menu on the left), click Edit Email & Password button.
Note: after email change you have to reactivate your account, clicking on the link that was sent to you. In case you miss your activation email, it is good to be in contact with someone at PF (not using PF PMs) so that you can inform us about problems.
Borek:
Hi, this is Bacle, from the OP.
I did not receive any e-mail to allow me to change my e-mail address. I opened up a new e-mail account, and I just registered here now under Bacle2; please let me know: what should I do?