What are the two blisters on F-35 upper wing surface? For all of the F-35 models you can find pictures with and without a blister on the upper wing surface. I'm guessing that it is sensors for the missile warning system that were retrofitted later on, but I couldn't find any information supporting that.
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