To begin with I would '''take it apart''' and '''clean''' the '''buttons''' and the '''area around them''' and see if that makes it better. '''Don’t''' use water for cleaning any electronic part, it may ruin it.
This will help you take it apart: [guide|127765][br]
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If that doesn’t help, you probably need to replace them. Didn’t found any replacement from a short search but if you spend some time searching, you usually find some.
'''Hope this helped and good luck! :D'''