Classical guitars have 6 nylon strings although 3 of them look like metal (these have a nylon core and a thin wire wrapped around to make them thicker). Classicals need to have their strings changed regularly to get the best tone and playability.

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.
P.S. NEVER USE STEEL STRINGS ON A CLASSICAL GUITAR! They can cause irreparable damage.
 

STEP 1

Firstly turn the machine heads clockwise to make them slack.


STEP 2

Cut all 6 near the sound-hole once you have slackened them all off.
 
 

 STEP 3

Us a pair of wire cutters or string snips to cut them.


 STEP 4 

Remove the string from the barrel. This can be tricky so take your time.


 STEP 5

Now remove the strings from the bridge.

 


STEP 6

If they're still too tight you can push the string through and unwind the loop.

 


STEP 7

Remove all 6 strings and give the bridge a good dust.


STEP 8

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.


STEP 9

Now give your guitar a thorough clean and polish with a different cloth as the lemon oil can damage your guitars finish.


STEP 10

Get your new set of strings ready. 


STEP 11

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.


STEP 12

Push the string through (from the sound hole side) the hole in the bridge. Start with the thickest string.


STEP 13

You will need around 3 inches pushed through. We will refer to this as the tale.


STEP 14

Take the tail, bend it back over the bridge and to the right of the string.


STEP 15

Loop the tail around the bottom of the string and back on itself.

 


STEP 16

Feed the tail under itself and pull the slack through.


STEP 17

Again, feed the tail under itself a second time.


STEP 18

Now pull on the longer part to make the knot tight.


STEP 19

Here is a close up of the knot.


STEP 20

Repeat this process again with the next string.


STEP 21

On the next knot take the tale and tuck it in.


STEP 22

Here is a close-up showing how the tail is tucked in.


STEP 23

Repeat with the rest. With the thinnest E bring it back in the opposite direction and tuck it under the B string.


 STEP 24

Pull the thickest string through the nut, ready for cutting.


STEP 25

Measure 6 inches (15 cm) past the required tuner roller (Low E is bottom right).
 
 

 STEP 26

Cut the string with wire cutters at the 6 inch mark.


 STEP 27

Pass the string over the tuner roller and then pass it down through the gap.


 STEP 28

Feed it from underneath and up through the hole.

 


STEP 29

Feed approx 3 inches (7 cm) through the roller to give us another tail.

 


STEP 30

Fold over the tail approx 1/3 inch (1cm).


STEP 31

Loop this little hook over the longer part of the string and put it back through the hole in the roller.


STEP 32

Pull on the longer part of the string  that passes down the neck to make the loop tight.


STEP 33

The next step is much easier with a string winder.


STEP 34

Keep the string under tension as you windup up all the slack and bring the string up to tension.


STEP 35

This usually leaves you with around 4 winds. Keep the loops looking like this (working towards the middle of the headstock).


STEP 36

Now repeat for the rest of the strings. You can now tune and stretch the strings in. Happy playing!