how to replace tension spring on sewing machine?

I have an older black sewing machine (made in Italy says Caser on the head, not sure if it is a Necchi) that needs the tension spring for the upper thread replaced on the tension dial. I took the individual disk off down to where I could see the spring but I'm not sure if I should pull the spring out or if I need to remove the entire shaft to get to the spring. Any help would be appreciated.

Tenna
August 2008