I was diagnosed with a substernal goiter, supposed to have surgery. How do they do it?

Neck incision. Usually it can be done thou a standard lower neck incision but sometimes the sternum bone has to be cut to get access to the upper part of the chest.
Through the neck. Most substernal goiters are readily removed through a standard thyroidectomy incision in the neck. Very seldom is it necessary to have an opening made in the upper part of the sternum. If it is known ahead of time that the goiter is very large and extends quite far into the chest, then a thoracic surgeon should be on standby or even potentially perform the procedure.