First thing to do is sight the neck by looking down it longways and see how the neck is warped. Then look for a place where the neck is joined to the guitar body for access to a female hex-nut hole to use a hex-wrench on. If it's an acoustic guitar with a soundhole, the hex adjustment ought to be just inside the soundhole near the neck-body end. If it's an electric solid-body guitar, the hex adjustment ought to be under a plate cover somewhere at body end of the neck, usually back of the neck near the body.
To adjust the neck bow, you loosen the string tension first; then turn the hex wrench in the slot so the neck starts bowing backward a bit, just enough to keep the guitar strings from buzzing when you tune them back up to standard tuning (get a tuning fork to tune the E-string or A-string; it's cheap; it's easy; it's good for ear-training).
You do not want to do a whole lot of adjustment all at once. Do it in increments. Adjust a little, tune up the strings to see if it's enough; do it a little more if not enough, like that until you get it where you want it. You only want a little bow under the strings, just so when you play notes at the first fret you don't hear them buzz. Fret the string slightly behind the fret, not in front of the fret.
If you don't have a tuner or tuning fork, you can make your strings too tight (bad for neck and where it attaches to the body). If it's an electric guitar, and the strings are too tight when tuned properly, the intonation of the strings is probably wrong. When you play a string open, you ought to hear that very same note at the twelfth fret one octave higher in pitch. On an electric at the body end where the strings terminate, the strings sit in individual 'saddles'. Each one usually has an adjustment up and down (height) and back and forward (to set intonation -i.e., length of the string).
If you've got an electric guitar, check the neck for warpage (slight neck bow under strings is preferred). If the bow looks OK, then adjust the individual string's saddle height at the body end where the strings terminate. That should take care of string buzzing.