Is it possible to change or add target organization on salesforce sandbox?
In order for change sets to be sent from one organization to another, a deployment connection is required between the organizations. Check the below link for more information.
go to your target instance , then Setup , which is located @ https://SFDC.InstaceDomainName/lightning/setup/SetupOneHome/home
Left hand search box , key in "Deployment Settings"
you will see list of connection there tell the relationship between instance.
Click "edit" , then check "Allow Inbound Changes" to accept upload
Now this target instance name will automatically populated on source instance "Outbound Change Sets" screen