You will always have the ability to edit the projections you set to your keywords ( in Step 3, Step 4). Even after you have reached the end, you can continue to edit your keyword projections until you get your forecast just right.
However, it is not possible to modify Step 1, which is the initial projection setup. Once you set the website you want to forecast, the competitors and the target keywords, your forecast is set.
The reason for this is because once you Step 1 is complete, we build your keyword database. Once your keyword database is set, you cannot edit it (due to the costs of pulling the keyword data into your database).
You will need to setup a new projection in order to edit the website in which you are forecasting.