You can change your password through the Settings section of the site, accessible by clicking your name from the top menu. Select 'Change' alongside the 'Password' line.
You will need to enter the current account password and then enter in your new password.
It is recommended that you use a strong password for your account that isn't used elsewhere. Strong passwords include mixed letters and numbers.
When does the change take effect
Password changes are processed straight away and your password will need to be entered in the future for you to be able to log into the site.