You can delete any weight measurements in the Limbo app, and manually add weight readings in Apple Health
If for some reason your starting weight is not correct, scroll in your main screen of the Limbo app to the weight entry, select it and 'delete' by clicking on the 'x'.
If the weight entry is too far in the past, contact the care team and they can do this for you manually.
To add a new 'starting weight' you can go into Apple Health, navigate to Weight, and Add Data, timing the starting weight to just after you logged onto Limbo for the first time.