To change your Magpi password, you'll need to access your account pages. To do this, first log into Magpi, then click on your username in the upper-right corner of the screen:

This will bring you to your account pages, where you can change username and password, upgrade your account, purchase more credits, etc:

At first, you will be on the Usage tab, where you can see your credit balance, upgrade, renewal date, etc.  To change your username or password, you will need to click on the User tab (see image above).

The User tab, below, allows you to change username, email, and password: