This is the freezer model GSU2320GB/52.
When I pressed the button below the centigrade sign, the temperature went up to - 21 degrees.
How do I get it down again to -18 degrees?
It is a 'no frost' freezer.

Emilie Erbmann, January 2015

I wouldn't worry - Minus 21 is colder (lower) than minus 18 but both are well below freezing - your frozen food will not notice the small difference.

Clarence Birdseye the father of Frozen Food liked round numbers and set his freezer temperature to be 0 Degrees F which is the same as minus 18 C. Well below freezing and purely arbitrary.

You could try pressing the button repeatedly until you get -18 in the window - my freezer has settings from -15 on the warm side down to -32 at the cold end.

Stay cool..., January 2015