Not hard if you have the specialized tools, knowledge and licensed to purchase the refrigerant. For a regular homeowner, it is not a realistic job to attempt and will usually cost at least $75+ to have done for you by someone qualified.
In most cases it is hardly worthwhile to have done to a dehumidifier.
How did the refrigerant leak out anyway?
The refrigerant in domestic refrigeration appliances (fridge, freezer, air conditioner, dehumidifier) travels in a sealed system and should normally never need to be 'recharged' or 'topped up' unless damage was done to the refrigeration system or the compressor needs to be changed.