And then select Update and Promote.
This is a two step process. Step 1 promotes the plan to the current planning year and Step 2 gives you the opportunity to update values. This is optional as you can update values by going into the Dashboard. 
Click on Next and it will take you through Step 1, which is promoting the plan and setting a new start date, and then you will move to Step 2 where you can up date the asset values. During Step 1 take note of any messages about items which will no longer appear in the plan due to their timing.
At the end of Step 1, click on Promote
A message will appear asking you if you are sure you want to promote i.e. create this new updated plan, click Promote again.
You will now see a screen that confirms Step 1 (promote) has been successful and now you proceed to Step 2, making the updates. Click on Next.
This will takes you through the various areas where you can make updates e.g. incomes where you have the opportunity to update the figures or delete. To move through the screens, click on Next.
Once you have worked through the update screens you will get a summary and then click Done.
Again a message will appear asking you if you are sure you want to update , click Update.
Once done, check the Dashboard and update any items not included in the Update wizard.
You can also use Let's See > Compare Plans > List View to compare the new base plan to the old one in order to sense check it:
Further reading:
How to add a what if plan in **AdviserGo**