Answering my own question:
thank you Jack Zimmermann
https://www.jackenhack.com/calibrating-brymen-bm-series-multimeter/amp/#ampChromeExtension=https%3A%2F%2Fwww.jackenhack.com%2Fcalibrating-brymen-bm-series-multimeter%2FThe calibration procedure is the same for a lot of Brymen and Greenlee multimeters.
First, turn the multimeter off
While holding down Select and Hold buttons, turn the selector dial to Ω within a second. If everything worked, you should get 250.000Ω on your multimeter display. If not, turn off and try again. It’s slightly fiddly to both press two buttons and rotate the dial.
After you get the 250.000Ω on display, rotate the selector dial to the part you want to calibrate, in my case, DC Volts.
When calibrating DC voltage, the first one is 0 Volts. Just connect a short banana test lead between the COM and VOLT socket to short out the input to give you 0 Volts.
On the BM869s, press the Range and Δ buttons simultaneously to get the multimeter ready. On other models, you need to push Range and the Hz buttons.
Press the Range and Hold buttons to store the value.
The display now shows 5.0000 V on the display. Connect the 5 Volt reference, while at the same time measuring with a good meter and adjust the potentiometer to get to 5.00000 Volts.
On the BM869s, press the Range and Δ buttons simultaneously to get ready, or Range and the Hz buttons.
Press the Range and Hold buttons at the same time to store the value.
The display now displays 50.000 V. Connect the power supply to the potentiometer, adjust the voltage to exactly 50 Volts.
On the BM869s, press the Range and Δ buttons simultaneously to get ready or press Range and the Hz buttons on other models.
Press Range and Hold to store the calibration value for 50 Volts.
If you have 500 Volts available, you can continue with the calibration, repeating the above procedure. Otherwise, you can just turn off the meter. The stored values for 5 and 50 Volts remains.