There are several ways of doing this, but first of all, can you get into PPrUne with your new password as well? This should not be the case!
Here for PPRuNe your password is actually cached with a cookie.
To remove it the easiest in Internet Explorer is to click on TOOLS --> INTERNET OPTIONS and than in the middle on DELETE COOKIES.
This way you delete all your cookies and you need to re-tyoe your password on any website where this procedure was followed.
There are of course other procedures as well but, to explain them, I need more knowledge about your operating system. What are you using and which browser?