How can I replace my strings on my First Act ME402?

Hello, My question is that I got my cousin a first act guitar for his birthday. He was playing it wrong and next thing you know the strings broke. (aCTUALLY ONE ONLY BROKE) I know I shouldve tuned it myself and showed him how to play it. I didnt. So know one string is broke. So I decided just to replace them all with better strings. So I go and get some strings. I come back to replace them. I pull out the old one and it seems like it jut went trough the whol and up the neck, so I did that and it looks perfectly fine. I feel like I took the short Cut and would prefer the proffesional way. Please Help me.