How to convert the forecast in a production plan

Summary:

We have a CPG customer, which has setup the business recently. They are growing exponentially.

I wanted to understand below points:

1. How can they extrapolate the forecast by a fixed percentage, if the system suggested forecast is low. As the system would suggest forecast based on shipment histories.
2. The forecast would be reviewed with production team to discuss the capacity. Currently there are capacity constraints. In certain cased, the forecast might be trimmed down. How can they make mass changes to forecast using excel sheet. If the plan runs again, how to make sure that the changes done to forecast are not overwritten.