Two possible methods...
Take the .FBX model into a program called Blender. Here you can add clothing, edit the body, essentially you have access the entire model with what you want to change.
To start, locate the .FBX within the Unity package, typically they have their own folder called "FBX".
If there isn't an .FBX within the Unity Package, you can convert a Prefab into one using the following method.
When the time comes to re-importing the model, in Blender, click Export as FBX and locate your Unity Project Folder. Find the FBX folder and/or .FBX file of the model and replace it (ALWAYS make backups just in case). You will need general knowledge of how to make those new clothing pieces toggleable if you desire. There are lots of video tutorials discussing these types of changes and how to do them.
For Booth models you can use 2 different Tools,
First tool - Avatar Tools
Second Tool - Easy dress-up tool
The following are mainly for Booth specific models. Any clothing that was made for that specific character, these tools will allow you to add them without having to use any other third party software. They must be made for that specific model/character or you may run into problems or it will not work. In a case where the first tool doesn't work you can try the other. They're both pretty straight forward, some basic knowledge will go a long way for making avatars in general.
In the case of non-booth models, there's a chance these tools can work as long as they're correctly named bones and/or weightpainted with a match to your characters body, though it's best to use Option 1 for that.
It is possible to add some things like most jewelry, hats, some shoes, etc. (basic accessories) without needing 3rd party tools or taking the model into another program.