There are two ways to edit a phase’s action buttons. If there is a card in the card you want to edit the buttons, open the card that's currently in this phase and click on the blue 'move to' button in the bottom of the right side of the card.
That'll open a list of the phases you can move this card to. Click on 'edit buttons' to edit those phases.
Once you click on it it'll open a pop-up window with the name of all your pipe's phases next to a switch. If the switch is gray, it means the action button is inactive and cards can't be moved to that phase.
If it's green, it means that the action button is active and you can move cards to that phase from the one you're editing.
Click on the switches to make the button active/inactive then click on save.
If there isn't a card in the phase you want to edit or if you simply don't want to open a card to edit the action buttons, click on the three dots next to the name of the phase you want to edit then choose the option 'move to phase' to edit the action buttons as explained above.