How to change inline water filter on whole home humidifier system?
This filter appears to be a single unit, so the way to replace it would be to disconnect this one and install a new one. First make sure the water supply to your humidifier is off.

The attachments look like 1/4" copper compression fittings. If you look at the fittings in top-down order on your second pic, you have first the copper tubing, then a nut which is clamping the compression fitting onto the copper, then a male adapter that threads into the filter inlet. To disconnect the filter use one wrench on the plastic "nut" at the end of the filter to hold it steady, and another wrench on the adapter (the brass part closest to the filter). Should be a normal counterclockwise unscrew operation.

Then you can reuse those fittings with a new filter. Alternatively, you can cut the copper tubing and install new fittings with your new filter.

You're probably looking for an "ice maker filter" like this: https://www.amazon.com/LASCO-37-1821-Compression-Connection-10-Inch/dp/B00HYXR24S/ref=sr_1_3?s=appliances&amp;ie=UTF8&amp;qid=1514872086&amp;sr=1-3&amp;keywords=water+filter+1%2F4%22