How to restring a guitar?

Strings are available from any music instrument shop. There are different types available which have different tensions and you need advice as to which your guitar has already. I would only change one string at a time as if you take all of the old ones off at the same time, you will release the tension in the neck, and I don't think that will do it any good. Take the guitar along to a reputable shop and seek advice. They will not charge for the advice, after all they are going to sell you some strings and retain a possiby good future customer.