What are the two blisters on F-35 upper wing surface?
Those are Luneberg Reflectors.

They enhance the radar cross section of the aircraft.

For two reasons; to be more safe around civilian traffic due to being easier spotted by traffic control radars, and where they do not want adversaries to train their radar systems on it to gain knowledge of its strengths and weaknesses.

Not sure if those on the picture is retractable, or has to be fitted/removed by mechanics.

There are many sources for this, here is one:
https://www.businessinsider.com/f-35-luneberg-radar-cross-section-russia-estonia-2017-5?r=US&amp;IR=T