Why adjust the truss rod?
Due to the changes in humidity and temperatures in the room where you keep the guitar the neck can bend. When you change your strings the chances are that the neck is going to bend because of the tension difference. So if you notice your neck is bent and you don’t like how it’s bend you need to adjust the truss rod.
Adjusting the rod isn’t too difficult, but it is a bit scary. I would recommend you try it, and if you are not feeling comfortable with it then take it to a guitar technician.
Guitar neck 
Guitar action
Where to adjust the truss rod
Every single guitar has a truss rod. On most acoustic guitars the access point to the rod is located in the sound hole. However on some models depending on the manufacturer it could be located at the headstock.
Usually when you purchase a new guitar you get hex key with it, there is no standard for which size you need. It varies from manufacturer to manufacturer.
Truss rod adjustment
Where to turn the hex key
When we place the hex key into the rod access point, depending on where we turn is where the neck is going to bend.
If we turn the key to the right, we will tighten the neck. Which means that the neck is going to have an upbow.
If we turn the key to the left, we will relief the neck. Which means that the neck is going to have an back bow.
Hex key to adjust the truss rod 
Placing the hex key to adjust the truss rod
Tightening the truss rod 
Neck relief
How you adjust the neck is completely up to you and your liking. However having a big back bow on your neck is probably going to cause fret buzz, which you definitely do not want on your guitar.
When you’re adjusting the rod make sure you don’t force it, only turn it in small increments. If you tried it and it feels like it’s stuck, take it to a professional and if possible definitely watch how they do it.
Turning the key to the right 
Turning the key to the left
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