This is because you have your environment set in IQA, the environment sets the endpoint. If you change the environment to default environment, you could manipulate it. Or add/edit the environment with the correct endpoint.
If im understanding your issue correctly, the greying/disabling of the endpoint field is correct behaviour. You are viewing teststep in your screenshot. You cant edit the endpoint of a teststep. If you need to alter the endpoint for a teststep you need to edit the field on the request within the service.
What i mean by this is that a teststep can be considered a child record of the parent service (the api) record and if you think about it for other things (db records, etc.), you cant typically edit a child record, you need to edit the parent and then these changes are reflected dynamically within the child record.
This is the same for teststep records within ReadyAPI