There are two ways, depending on whether you still know your current password.
You know it
Open your dashboard at me.melodreams.com, go to Account, then Password. You enter your current password once and the new one twice. If two-factor is on, you will be asked for a code as well.
You forgot it
From the login screen, hit "Forgot password". We email a reset link to the address on your account. The link expires, so use it soon after it arrives, and ask for a fresh one if it has gone stale.
Your other devices
Changing your password does not silently sign out your other sessions. If you think someone else is in your account, sign them out yourself: Account, then Devices and sessions, then sign out everywhere. Do that after the password change, not before.
Picking one
Long beats complicated. A passphrase of a few unrelated words is stronger than one short word with symbols glued on, and you will actually remember it. Do not reuse a password from another site. That is how most accounts get taken, and nothing we do on our side helps if the password already leaked somewhere else.