You will be able to edit the manually filled-in data for the relevant date again, with the right information, within a 10 day window. For example, for a run data for May 5th, you can edit the data latest by May 14th, and no later.
Any auto-imported run-data from your Strava account cannot be edited by the runners. If the runner feels that any one of their auto-imported run-data isn't correct, the runner will first be required to delete that run-data from the 100 Days of Running account after logging-in and then manually enter correct data within the prescribed time limit. Again, needless to say, you will be able to do this within the 10-days window. For example, if there was issue with your Strava imported data for May 5th, then you can delete and manually reenter the data latest by May 14th, and no later. Do note that all rules related to manual entry will apply, like no entry more than 10 km allowed.