Convert Right-Handed Classical Guitar To Left
1. Remove the strings. You can save the strings for later if you don't have new set of strings yet. But if you do, just chuck old strings away.
2. Put strings in reverse order. Top string goes to the bottom and the rest will follow.
3. Your low E string which is your 6th string goes to 1st or your high E.
4. Your A string which is your 5th string goes to 2nd string which is your B.
5. Your D string which is your 4th string goes to 3rd string which is your G.
6. Your G string which is your 3rd string swap places with 4th string which is your D.
7. Your B string which is your 2nd string swap places with your 5th string which is your A.
8. And, obviously, your high E string which is your 1st string swap places with your 6th string which is your low E.
NOTE: Because this is the quickest or the simplest way to convert your guitar, there are other stuff that's important to know like the nut and the bridge. In some cases you will have to turn it around as well to accommodate the string thickness. However, most acoustic and classical guitar are fine apart from the electric guitar which is a bit trickier.
Hope this helps.