If you don't know your password, you'll need to recover your account, see our Recovering a Matrix Account page for instructions.
Go to Element All settings
You might want to export your E2E room keys. Just to be on the safe side in case something goes wrong. See also Export and Import E2E Room Keys
E2E room keys
Enter your current password and your new password
You now need to sign in again on all your other devices
If you're an EMS customer, you can create your users via the Server Admin tab of the EMS Control Panel.
Alternatively you can make use of the Synapse Admin API to change a Matrix Account password on a homeserver you hold an Admin account on. To do so, you will need to use Create or Modify Account from the User Admin API.