Formaldehyde safety levels
Two different parts to the question I think:


Are the readings you are getting on your meter meaningful?
If formaldehyde levels are high, what (if anything) can/should you do about it?


Regarding interpreting the reading from the meter, there are a few things you should consider right off:


What does the manufacturer say about accuracy? Typically the results should be what's displayed +/- some amount (say 25%). If this isn't documented, you may be able to contact the manufacturer for more information.
What other substances can cause interference with the sensor. It's very possible that there are other contaminants in the air that are causing the reading to increase. Again, check the documentation and/or contact the vendor.


My experience (especially with non-professional grade equipment) is that direct reading instruments are typically good at helping to understand concentration trends. For example if you check the readings over the course of the day, you may find higher concentrations at certain times and lower concentrations at other times - could be linked to temperature, ventilation rates, or something else. The information is useful for understanding at a high level what's going on and for figuring out why you have a problem.

I have a lot less confidence in the absolute readings that they provide. Professional grade meters for formaldehyde can cost thousands of dollars and require regular calibration. And even then, if I wanted to be confident in my results, I'd prefer to use a wet chemistry method like one of these.

Now, assuming you are still convinced that you have a formaldehyde problem, what can you do about it?

Some people will say that you can attempt a "bake-off". Get some space heaters and run the indoor temperature up to as high as you can get it and keep it there for a day or two. The idea is that heating everything up will cause whatever formaldehyde reservoirs to fully "off-gas", and when the temperature returns to baseline, there won't be any formaldehyde left. I'd be cautious on following this method, however, as some studies have found that any reductions seen are generally short lived.

Three more realistic approaches are:


if you can increase the amount of outdoor air coming into the house, this will go a long way toward diluting concentrations. 
NASA has also done research and found that certain types of house-plants are good at absorbing formaldehyde from indoor air.
Now that you're more aware of formaldehyde as a potential indoor air quality problem, keep it in mind the next time you update furniture or your home interior. There's increasing awareness in the market, and you should be able to find low VOC formaldehyde-free alternatives for most relevant products.