Update: The game contains a "content manager" inside the game which you can use to install/uninstall store content and cars/tracks.
There are a couple of ways to install content for rFactor 2. One is by using Steam Workshop the other is by using the Mod Manager that comes with the game and can be found in your "rfactor 2/bin64/" folder. Look for the ModMgr.exe.
Mods that come as a file (*.rfcmp) will use the Mod Manager to install/uninstall.
Be aware that if you uninstall mods with the Mod Manager that you subscribed with Steam workshop, these will be reinstalled again when the game restarts. Unsubscribe on Steam workshop for these mods.
Run the application and you will be presented with all the content installed in your game. I will take you through the essential parts first to get you setup correctly.
If you look at the bottom right you will see the Working Dir and the Packages Dir. Firstly press the B next to Working Dir and choose your rfactor 2 root folder. Mine is shown in the box, your folder structure may differ.
The Packages Dir can be set to any folder which you have your mods (cars, tracks etc...) contained. I have several for my cars or tracks. You can even set more than one. By default it's best to set to your Packages folder ("rfactor 2/packages"). If it isn't present, create the Packages folder in the rfactor 2 root folder.
If this is all set correctly then all the content installed in your game will be shown in the main window.
On the top left you will see 2 checkboxes. "Mods" and "Components".
"Mods" - Slightly confusing but this refers to *.rfmod files which are commonly known as a "Series". A "Series" is a group of cars and tracks mostly used for hosting a dedicated server or using rules to dictate a season.
"Components" - These come in the form of *.rfcmp files which are basically archives containing all the relevant files needed. These are any in game content such as car, tracks, sounds etc. Once installed, these files can be deleted but sometimes best to archive them somewhere just in case you want to reinstall them at a later date.
Along the top you will see many 3 letter abbreviations for components. These can be turned on and off to suit what you want to see in the list.
The main ones are:
"Loc" - Location
"Veh" - Vehicles
"Snd" - Sound
"Tln" - Talent
"Shr" - Showroom
The refresh button is used if you have the app running when you downloaded a mod, you can refresh the list so it will appear.
As you can see from my screenshot I have mods that are installed already, and some which are not. The Yes/No in the installed column indicates this. When you highlight an item the details will be shown at the bottom and the Install/Update/Uninstall/Delete buttons will enable/disable depending on what you are able to do.
You can use multiple selections to Install/Uninstall/Delete many at once. Hold ctrl and click separate items.
These are made with the Mas2 tool which comes with the game, grouping installed content together to run on a dedi server or your own single player races with specific rules (e.g. pacecar etc...). These are basically an anti-cheat method to make sure everyone on a server is using the same content.
At some point you may want to uninstall some content but run into problems. If content is in use by a series, it cannot be uninstalled until the series has been uninstalled first. See the status "In use by X mods" will give you a clue about this. Then trying to uninstall the content will pop up the error pictured on the right.
This also means that no content can be updated if there is a series made with it. It's sometimes best to uninstall the Series you are not using.
You may have a few of these installed from when joining random servers.