You can't retrofit ships (Stellaris)

Macau on the flotilla → manage fleets, Macau for each type of ships (and why it is written x 0 each type) → retool, Macau on the desired drawing (void)



Now, again jmackay on the flotilla, under the fleet on the bottom click on the improve (⭡?), and ships NOT improve until the drawing that I chose in the menu of the flotilla (Antica instead of void).