While starting Tails, set up an administration password .
Open Disks from the menu .
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 contains the persistent volume. It should have two partitions, one labelled Tails, and another one labelled TailsData which corresponds to the persistent volume.
In the right pane, click on the persistent volume partition, labelled TailsData.
Click on Change Passphrase….and choose
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 enable the persistent volume with its new passphrase.