If you want to, you certainly can. I'd suggest tasting it first. If the beer is fine the way it is, I wouldn't mess with it. If you taste it and you think it would be better watered down, then add the water.
If you were too high on your OG when you brewed, you could have added the water then. That's what I do. Check the OG as the beer goes into the primary. If it's where I wanted, I don't add any water. If it's too high, I add water until I get the OG I planned. After that, I don't add any water to it to make up the losses to trub or racking.
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