Hi, @Cherjade! I believe they took away the actual ability to do this a while (weeks?, months?) ago in iOS. It's been gone for years in Android. You can edit in iOS, but it reverts back to the original.
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I’m having the exact same issue. I used to edit my swim workout yards due to the Charge6 not picking up distance when using a kickboard. In my IPhone app I edit the distance and click done it changes the yards swam for about 1 second and then instantly reverts back to the original distance. This has worked for me for the last year on multiple fitbits, now after this last update no such luck. Fitbit please fix this bug or Apple Watch gets my next expenditure