When accepting per unit of effort pledges, pledges are calculated based on the estimated units for a participant. Administrators start with Average Units per Participant and participants can specify their personal estimates (i.e. laps, miles, strikeouts).
You can define the Average Units per Participant for all participants under Pledge -> Settings. Participants have the ability to change this from their Participant Control Panel.
If you want to update the estimated units for a particular participant, 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'.