It's always advised to complete a risk assessment from scratch, however if you want to copy a already completed risk assessment then you can.
It's quite easy to do so
Go to risk assessments
Go to completed forms and click on the edit / view button to see the form
Click on make a copy of this form (its right next to the down load button)
OK so its copied now and the form you see is the orginal
Click on risk assessments
Go to completed forms
You will now see you have 2 forms with the same name.
Open up the clone and edit the name so you don't mix things up.