So,recently I’ve downloaded the 4.15 version.But I have a problem now with my project:the map is too lit.Let me show you:
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thats partially due to the new tonemapper and a few other things. (which is why you always need to read updates to ue4 to know what to expect)
it means you will need to re-tweak your lighting/post processing volume or stay with the previous version.
(also, rebuild your lighting)
So how can I re-tweak my lighting (I’m pretty new to ue4)?I can’t go back to the previous version due to a bug that was fixed in 4.15
So,I found the problem(I think).I was using level streaming and all I needed to do was to rebuilt the lightining with all the levels visible 
great!
also, please accept my answer as correct. that way people can find the right answer easily in the future!