Guzheng is tuned to a pentatonic scale. Tuning means establishing a reference pitch first, then adjusting each string against it. The reference usually comes from standard concert pitch, or from the ensemble or accompaniment.
Beginners find an electronic tuner or a phone app much easier. Place the tuner near the instrument, pluck the string and read the note name and the sharp/flat indication, then adjust with the tuning pin.
Adjust in small steps and keep the motion steady. Turning the pin too far overshoots and forces you to come back; pluck with consistent force as well, since varying it makes the tuner reading drift.
Once the pitches are familiar you can move gradually to judging by ear. A common route is to tune one group of strings with the tuner and then tune the rest by listening.
New instruments, freshly strung instruments and periods of significant environmental change all make pitch unstable. Several small adjustments are needed before it settles — this is normal.