If not secured, nylon strings can easily slip. Here's a brief video showing you a secure way of stringing a classical guitar.
It's good to change the strings at least once a year. Some professional player change them every month, or even every few weeks.
New strings will make your guitar sound like a 'new' instrument. Here's a short video giving showing you how to change nylon guitar strings. It has a bit too much reverb, but it should help you with learning to change your strings.
