1. Since you don't know how, then DON'T do it yourself. DANGEROUS
2. You need tools
3. When one broken, the other one will break pretty soon. You have to change both.
** Usually a spring can be used to open garage door 10000 times.
I hired a guy and he spent aournd 1.5 hours to change both springs. He started at 11:00 p.m. and finished around 12:30 a.m. He has the tools. Cost of two strings is around 60 - 70 plus tax. With parts and labour I paid 200 CAD cash.