Let's assume Tstart = 100 ºC, Tfinal = 105 ºC.
Delta T = Tfinal - Tstart = 105 ºC - 100 ºC = 5 ºC
Now in Kelvins: Tstart = 100 + 273 = 373 K, Tfinal = 105 + 273 = 378 K.
Delta T = Tfinal - Tstart = 378 K - 373 K = 5 K
No matter whether you use K or ºC, answer is the same. Take a look again:
Delta T = Tfinal - Tstart = (105 + 273) K - (100 + 273) K = (105 - 100) K + (273 - 273) K = 5 K
273 just cancels out.