With the limited context provided, this is the best I can give you (more info about what the reviews are for, what they contain, and who the users are would be useful):
I consider those two different usecases.
Amending a "bad" review (I would encourage you to rethink the wording, though) with a change, explanation, new rating, ... is great for transparency. In the case, that the user needed your input to realise their "mistake".
What if a user posts something by accident? If they mistype, if they do something wrong and post without noticing? What if they realise they misunderstood something and want to delete the irrelevant and "bad" review? They have to write to you? I find that uncomfortable for the user (bad for them), and also much less likely to happen (bad for you).
Regardless of that, I believe users should be able to delete anything they create / post. And in at least some cases, this is also legally required - did you check that?