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It is somewhat challenging to determine how to set the barometer without actually looking at it but many aneroid barometers which is most likely the type you have are set by using a small screw in the back. You can determine local readings from another barometer with known accuracy or possibly from the news and then set yours accordingly.
There are two types of pressure to consider...
Station pressure and sea-level pressure
Station pressure is the actual pressure at your location
Sea-level pressure is the adjusted pressure depending on what your elevation is.
The local news should be providing you with sea-level pressure.
Your barometer most likely has units of in Hg which is inches of mercury.
Normal pressure is 29.92 in Hg
Anything below that is considered low and above that is considered high.
As a general trend lower pressures tend to bring poor or wet weather and high pressures tend to bring good or dry weather. You may even see descriptive words such as "fair", "stormy", or "dry" on your barometer as indicators to that.
If you have a small dial on the front with a small arm you can set that to the current pressure reading, then, as the pressure changes you can track if it is going up or down and how fast it is changing as well if you record the time of your readings. If pressure is dropping quickly it can mean bad weather coming and if it is rising quickly it can mean bad weather going away and better weather on the way in. If it remains steady, your weather will tend to persist as it is.
These are some tips in a nutshell but hopefully it was helpful information. Let us know if you need anything else.
Good luck on the manual. Since the company is no longer in business it will be a challenge.