how do you treat Heat rash?

Mahala
Errggg Girls 
I have bloody heat rash it's all over my arms my right arm is the worst. 
I'm naturally tanned so it's not that bad looking but it's awful because I can see it. 
Lovely English weather has been hot & cold on and off with sun very hot on good days. 
I got sunburnt on my shoulders and this rash appeared just after. 
How can u treat it?