skothing

Just bought a nelco r2000 at a church sale for $5. It was running and had a good stronger sound from the motor. It came with a manual, 11 feet and 36 bobbins in a cabinet. It has a certificate of purchased in September of 1963. That's why I bought it. That was the year my husband and I met. I cleaned it up and let the oil devolve for 2 days. Now it sounds better than ever. I'd like to change the tension spring but not sure how. Any ideas? Thanks