Estimates > Approve or Decline Estimate
Reminder that Change Orders only occur for estimates that have already been approved. You can make as many changes as you wish to an estimate that has not been approved by your customer. Once approved, you will be forced to create a Change Order which basically creates a new estimate to be approved.
In order to approve or decline an estimate, simply select the Approve or Decline icon as shown below.

Upon approving an estimate, the Approve icon will change to allow you to make Change Orders if needed.
NOTE: Remember that estimates may be approved directly by your customer if you use the customer collaboration portal.
There is no way to re-open a declined estimate. It was done on purpose for the reason that the document has simply been rejected. Keep in mind, that in addition to the above able to be approved directly by your customer, the customer could directly also decline. If the customer now wants to approve the estimate then you need to go back to the same Job and create a New Revised Estimate. You create a new revised estimate under the same job # and then import all tasks from the declined estimate to the new estimate. You will then see two estimates tied to the Job, the original one that was declined and the new one that was approved. It is the simplist method to have an easy audit trail for all the permutations of events with estimates, multiple estimates and revised estimates.