After finding out, Yongguk would just hug you endlessly. He would feel bad, that you had to go through something, which obviously affected you so much. You would have to promise him, that the scars were old and you were no longer doing it.
Himchans main goal would be to reassure you, making your feel good. He would tell you all the time, that this doesn’t change the slightest how he feels about you. Also he would try to keep an eye on you in secret, just in case your behavior changed and you would do it again.
Same as Yongguk, Daehyun would make sure you were no longer doing it. Daehyun would also want to know how long you did it and why you did it. He would feel slightly disappointed, that you didn’t tell him, but at the same time he would understand why you kept it secret.
I feel like Youngjae would have noticed your scars already some time ago, but waited for you to tell him. Self harm is a serious topic, so you need to take that step by yourself and not being pressured by him. Youngjae would accept it and try to move on with you, only making good memories.
Jongup would simply be understanding. At least he would try his best to understand the situation you were in. Similar to Himchan, Jongup would want you to tell him from now on everything that is bothering you, so that he can immediately be there for you.
Junhong wouldn’t really know what to do. Could he tell you everything will be alright? He would bet you already heard that sentence 1000th times. Junhong would want to keep you more save than ever and from time to time even look at the scars. It would be reassuring for him, that there weren’t any new ones.