I accidently opened a java file with something else, How do I change it back?

A few months ago I recently got Minecraft for PC and I was doing al these things with forge and mods when I accidently opened the java file thing with something else. By java file thing its when you download a mod and it shows the java symbol. So if anyone knows how to turn it back please help me it didn't bother me at first, but now its important and I need to change it.

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