What can I do about spark plug?

What is your question?

Guess who ever had car before rigged one of spark plugs. Looks stripped. Had hard time taking out but finally got it out. Tried to put new plug but won't stay. Put old one back in & it stays but it looks as it needs to be replaced. Any ideas on what to do? Replaced others with no problems. So has all new but one. So leave as is or will it do more harm?