setup (gear wheel) --> gear wheel ---> gear wheel : headline /caption is "Calibration"
move the focus with the arrow up/down buttons to the REL Barometer field and enter the value needed (ABS Barometer + Offset)
e.g. ABS Barometer = 960 hPa, offset is 40 hPa => the number which has to be in the REL Barometer field is 1000 hPa.
once the focus is in the field the first digit will be bolder than the others - change the digit by using the (+) or (-) buttons
move to the next (or another) digit by using the right/left arrow button
once done, just move away from the screen by pressing either the gear wheel button or the return button.
it is best to do the calibration in hPa, even if you later on prefer inHg. Change the barometric unit to hPa before doing the calibration. You can set it to inHg after the calibration is completed. Using the hPa unit allows for more accuracy.
As the console has no API, a setup via an app is not possible (only weather networks or custom server can be done via app)
To get your proper offset, read
https://www.wxforum.net/index.php?topic=40730.0 section 5
Good luck
By the way - the process is described in the manual page 66/67 onwards. The language may appear a bit tricky, but in principle it is described there (chapter 4.12 Calibration mode).
https://osswww.ecowitt.net/uploads/20220803/HP2551%20Manual.pdfI'm assuming that the whole configuration of a HP2561 works the same way as for the HP2551 - should this not be the case, my apologies
In my understanding it's only a hardware upgrade (or mainly canvas upgrade apart from the inbuilt T/H/P sensor) and not a software upgrade.
I just checked the HP2560 manual, and there the calibration (same text as HP2551) is in chapter 3.5 Calibration Mode