Start Tails and set up an administration password .

In the Welcome Screen, keep the Persistent Storage locked.

Open Disks from the menu Applications ▸ Utilities ▸ Disks.

Disks lists all the current storage devices in the left pane of the window. When you select one of these devices, detailed information about it is displayed in the right pane: its partitions, its brand, its size, etc.

Find which device is your Tails USB stick. It should have 2 partitions: one labeled Tails and another one labeled TailsData, which corresponds to the Persistent Storage.

In the right pane, click on the Persistent Storage partition, labeled TailsData.

Click on and choose Change Passphrase….

Type your current passphrase and then type twice a new one.

Finally confirm by clicking Change.

In the confirmation dialog, enter your administration password and click Authenticate.

You now can restart Tails and try to unlock the Persistent Storage with its new passphrase.