When a participant registers, you do not want them to have the ability to specify their personal estimates (i.e. laps, miles, strikeouts). You want to know if you can fix the amount specified so that it cannot be changed.
You can define the Average Units per Participant for all participants under Pledge -> Settings. Participants still have the ability to change this upon registration or from their Participant Control Panel.
For those that have already filled out their estimates: If you want to update the current estimated units for those who already entered something, you will need to do it from their participant control panel. This can be accessed from People -> Participants -> Click on the person and find the button to their Fundraising Page. On the Personal Settings screen, you can update the field 'Units Estimated'.