Choose ··· → Duplicate budget inside a budget, or Duplicate in a row's menu on the budgets list. Give it a name and tick Include actuals if you want them too.
The copy is a new draft budget in the same workspace. To start from an earlier version instead, open Version history and duplicate a snapshot.
On the free plan, a duplicate counts as one of your three free budgets.
Not what you were looking for? Write to hello@actualsbudget.com.