In Unreal Engine 5, the Niagara plugin is installed and enabled by default. However, if you are unable to see the Niagara options then you can check to make sure the plugin is enabled.
How to Enable the Niagara Plugin
First open the Plugins menu by going to Edit > Plugins.
In the Plugins menu, locate the FX section. You will see Niagara listed in this section. If the checkbox is checked, Niagara is already enabled. If the checkbox is unchecked, click on it to enable Niagara.
If you had to enable Niagara, it will prompt you that a restart is required. Click Restart Now to relaunch the Unreal Editor with the Niagara plugin enabled.
End Result
Once the UE4 Editor has restarted, when you right-click in the Content Browser you will have a new FX section that will have all of the Niagara options.