Sometimes a customer might request a variation or update to a Quote after it has been accepted.
To edit an accepted Quote you will need to undo the acceptance so that you can make the changes. To do this, hit 'Actions' then select 'Accepted ← undo'. You'll then be able to either edit the Quote and resend it for your customer to accept, or accept it on behalf of your customer.
If you want to avoid editing the Quote, the best way to handle a variation would be to create a new Quote that reflects the changes.