Assuming you are already logged in with a current password, click on Account Settings
in the upper right hand corner of the community, then select Edit Email & Password
from the left side navigation.
You will see a page that looks like this. Step 1
Enter your current password to continue. Step 2
Select a new password that complies with the rules to the left and then enter it a second time. The red X's must turn to blue checks. If they don't it means you are missing something there. Once this is completed, and the passwords match, you should be able to click to save your changes.
A good piece of advice, to help in remembering the more complex password, is to do a phrase instead of just randomly adding in the extra requirements.
That will stick in your in your mind better.
If you need further help, let me know.