When a player first joins a game, they are presented with a choice. They can import a character from their vault, or they can create a new character. If there's a template for your current system, then the template will be applied.

If your player chooses to create a new character then changes their mind and decides to import a character from their vault, they can remove the character using the Manage Character menu and selecting the Delete option. There isn't currently a way to have a character import more than one character under their control, for instance if they wanted to summon a familiar.

Worry not, because you as the GM have the power to solve this.

First, you'll need to unassign their current character from them. You'll need to do this from the character sheet window. Their token will be "In the party" from the Character's Tab in the GM Sidebar. Click this character, make sure it's the character assigned to the party member, then unassign them as the player.

After this is done, the player will instantly see the window asking them to choose between creating a new character and importing from vault.

Make sure they choose to import and that they select the right character to bring into the game. That's all there is to it. The new character will be their new primary token. You can reassign them to their previous token and they will have control of both.