You received the master password when you created the account. It should be in the file that you received when you created the account. You received 4 passwords.

Posting password
Active password
Memo password
Master password

CHANGING THE PASSWORD:

If you have the Master Password for your account, you can get access back to your account and change all passwords.
Log in to your account with your master password and then go to these sections to change the passwords:

On HIVE.BLOG: WALLET > KEYS & PERMISSIONS
On ECENCY.COM: SETTINGS > PERMISSIONS
On PEAKD.COM: ACCOUNT ACTIONS > KEYS & PERMISSIONS

Once you change the password, save the new Master Password and all four Keys (Owner, Active, Posting, Memo) that you are given.
Then store that Master Password in a safe place, preferably not on your device, but rather on a pen drive, write it on a piece of paper, whatever you need to do to make it safe, where you will never lose it.
Then, only use your Posting Key to login everyday, as that is the safest login method.
Even better: use Keychain or Hivesigner as a login method so that even your Posting Key will be secure.