As Humberto stated, it should be easy; but depending on the complexity of the design, and how well it is implemented in the application, it could be longer: the worse the implementation (more probable for lower quality apps), the longer it will take.

Therefore, if the app itself is well-designed and follows best practices, the cost should be relatively low at about an hour or two.

However, this does not take into account ancillary work needed to re-publish the new app, which itself could take an hour, two, or more, depending on the issues that may arise.