Hi, Firstly you need to find visual studio installer in your machine wich is by deffault in this path: C:\Users\MY-NAME\Downloads\Programs. Then open visual studio installer you will find your installed version of visual studio IDE, Press on modify go to desktop&mobile and choose mobile development with .NET. Go to the right corner and press modify.
Screen Shoots:
I hope this's the answer for your question.
Regards.