The nylon, when under tension starts to stretch. This makes it more prone to snapping as it causes micro-fractures in them. You will find that when they get old, it will be difficult to keep in tune and they may snap.
The bass strings (the ones that have a metal winding wrapped around them) show you more obviously that they are old and need changing. These are usually silver-plated copper, so if the copper is starting to show through, this is a good indication that they need changing. They can also turn black due to the silver oxidising, again another indication that they need changing.
In between the windings, the wrapped strings get gunked up with dead skin cells, oil from your fingers, dust and lots of other yucky things. This causes the string to vibrate unevenly and so not give a good ring when plucked. You may also find that your fingers smell funny after you have played.
If you have broken one, this is another indication that you need to change all 6 of them, so that you get an even tone.
Below you will find a step by step guide on how to change the strings on a classical guitar.
Firstly turn the machine heads clockwise to make them slack.
Remove the string from the barrel. This can be tricky so take your time.
Now remove the strings from the bridge.
If they're still too tight you can push the string through and unwind the loop.
Remove all 6 strings and give the bridge a good dust.
Add some Lemon oil to another cloth. Wipe it all over the fretboard but leave it to soak in for 30 seconds to hydrate the wood. This will also help soften any dirt and grime for a deep clean. After 30 seconds wipe the fretboard dry.
Now give your guitar a thorough clean and polish with a different cloth as the lemon oil can damage your guitars finish.
Get your new set of strings ready.
Sort the new strings, ready for installation. Take note of the system used to identify each string and what order they need to go in.
Push the string through (from the sound hole side) the hole in the bridge. Start with the thickest string.
You will need around 3 inches pushed through. We will refer to this as the tale.
Take the tail, bend it back over the bridge and to the right of the string.
Loop the tail around the bottom of the string and back on itself.
Feed the tail under itself and pull the slack through.
Again, feed the tail under itself a second time.
Now pull on the longer part to make the knot tight.
Here is a close up of the knot.
Repeat this process again with the next string.
On the next knot take the tale and tuck it in.
Here is a close-up showing how the tail is tucked in.
Repeat with the rest. With the thinnest E bring it back in the opposite direction and tuck it under the B string.
Pull the thickest string through the nut, ready for cutting.
Pass the string over the tuner roller and then pass it down through the gap.
Feed it from underneath and up through the hole.
Feed approx 3 inches (7 cm) through the roller to give us another tail.
Fold over the tail approx 1/3 inch (1cm).
Loop this little hook over the longer part of the string and put it back through the hole in the roller.
Pull on the longer part of the string that passes down the neck to make the loop tight.
The next step is much easier with a string winder.
Keep the string under tension as you windup up all the slack and bring the string up to tension.
This usually leaves you with around 4 winds. Keep the loops looking like this (working towards the middle of the headstock).
Now repeat for the rest of the strings. You can now tune and stretch the strings in. Happy playing!