You'll be able to set another bounty after the present one expires or is awarded, subject to the restriction $$\text{new bounty}\ge \min(\,2\cdot\text{previous bounty}, 500\,)$$ At present it is not possible to change the amount of the bounty after it was set. And I don't see a convincing reason to add such as feature. It's one more link to include in the interface, one more dialog window to design, and more code to maintain.