French has loads of phrases that use faire, this one is used to mean “make a U-Turn”. It can be used in exactly the same ways as in English; to either physically turn around or to “change your mind about something”.
Technically, faire un demi-tour is used with the physical U-Turn, however, in general usage the article (un) is often dropped, and the phrase takes on the meaning of to turn back, or, turn around. When the article is included, the U-Turn is the specific 180 degree movement that is often made in a car.
To conjugate faire demi-tour simply conjugate faire as normal and leave the demi-tour alone.
La route a été bloquée, donc on a fait un demi-tour – The road was blocked, so we made a U-Turn.
Je faisais demi-tour quand Je me suis écrasé contre un réverbère – I was turning around when I crashed into a lampost/street light.
Plus tôt il m’a dit qu’il allait laver la voiture et puis, il a fait demi-tour en me disant qu’il ne voulait pas le faire – Earlier he said he was going to wash the car, then did a u-turn by saying that he didn’t want to do it.