In game
At first go to the quest tab and look for the Password changer.
This shows up:
You have easy, medium and hard passwords, just type the password and see what it says.
(easy)
(medium)
(hard)
If you are 100% sure on what the new Password has to be, then Click Change.
If you clicked Change, the new password shows up in chat.
Forums
You must be logged on your forum account. Click your name on the top left and select account settings.
Hover over to Password and type in your current password and your new password twice.