Basics of how to change your guitar strings:
Just unscrew the keys to loosen the tension and then remove the attached part of the damaged string. Remove it out of the threading hole at the back and dispose of it. Take a new string and thread it in at the back; thread the tip (the back has a small round piece of metal attached to ti for anchorage) through the hole on the key and wrap it in in place. Now just raise the tension and tune it to your guitar.
A larger article of string maintenance has already been posted.
Search the net for detailed video guides.
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