Edit manually entered data in Health app

Yes, there is in a way: you can delete the wrong entry and create a new one.

I'm going to use the example of manually entered Blood Pressure:

1. Open the Health app
2. Tap Blood Pressure
3. Scroll down to "Show All Data" and tap it
4. Tap Edit
5. Tap the "-" icon to remove the wrong entry
6. Tap Done

At that point create a new entry.

You are welcome! Deleting data can be nerve-wracking. In the future, if I may suggest, try backing up your phone before doing so. It will give you some peace of mind, knowing that, if anything goes awry, you still can recover your data.