Further reading: https://en.wikipedia.org/wiki/Disk_encryption_theory
When you encrypt a disk, the data is re-written in an encrypted format. This data is encrypted using a long, secure key. The key is written to the start of the disk. This key is then encrypted using your given password. The password is stored nowhere except your mind.
You CAN reset the password (through some implementations) by re-encrypting the long secure key on the disk using a new password. This requires knowing the old password, to decrypt the key in the first place.
If you don't have the old password, you can't change it, as you don't have any way to decrypt and re-encrypt the long secure key.