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enter image description hereHello, I seem to be having trouble with opening Software Center, when I click on it nothing happens, I'm new to elementary OS and Linux. I think I did something wrong when I tried to update a repository.

I get a lot of "404 Not Found" when I run "sudo apt-get update && sudo apt-get upgrade", it ends with "E: Some index files failed to download. They have been ignored, or old ones used instead."