Blender is a free, open-source 3D modeling and composition software which is universally used to create and modify 3D models. Blender software is sufficient for modding, and with the implementation of Arma Reforger Blender Tools, it is the best software available to accomplish this task.
Text is not a great way to show how to use Blender, however, several tutorials exist online, and on YouTube, so don’t hesitate to search for answers if you get stuck or things aren’t working quite right.
Open Blender on your computer. The default new project will generate in your Scene, and you’ll need to delete the default cube, camera, and collections. Just click each item and press delete, or right click and click delete in the context popup.
Depending on the model type, you should be able to go into Blender > Import > [Model File Type] and select your model.
After import, the hard work begins. The page linked below goes over basics of model orientation, asset preparation, etc. Consider that page the source, and what follows on this page as extra - useful - information. The official tutorials are good for cars, but additional notes are below for helicopters.
https://community.bistudio.com/wiki/Arma_Reforger:Car_Creation/Asset_Preparation