How do I change the signing order after sending it to the recipients?

Thanks.

Is viewing it considered to be an action?

​ Depends on the Recipient Type assigned to the Recipient. In a scenario where a Recipient is set to "Needs to View" then viewing the envelope would complete the action and would disallow replacement or changes to the document. If the Recipient was set to "Needs to Sign" and they viewed the envelope but did not finish actions and selected "Finish Later" then the outcome would not be the same, as they did not complete all the required actions.

Can you please confirm that i understand it correctly that if people have signed, you cannot change (remove) the signing order?

The Signing Order can be changed for any Recipients who have not yet taken action but not for those who have already taken action such as Signing. So if you have a single envelope sent to 10 Recipients each Recipient with a Signing Order value that is unique.....1....2...3.....4 etc, and if that envelope was completed by the first 5 Recipients, then you still could correct the envelope and change the Signing Order or remove the Recipients who were listed as 6,7,8,9,and 10 but you cannot modify the Recipients who already completed actions who were set as 1,2,3,4 and 5.