Follow these steps to use a 3.5 inch touchscreen on your rPi.

1. Update your system with sudo apt-get update && sudo apt-get upgrade
2. Start raspi-config and go to Interfacing Options and enable the SPI interface.
3. Download LCD-show van Github:
   cd ~
sudo rm -rf LCD-show
git clone https://github.com/goodtft/LCD-show.git
chmod -R 755 LCD-show
4. Enter the folder you just cloned with cd LCD-show
   Now all you have to do is start the script for your LCD type.
   For a 3.5″ display you run sudo ./LCD35-show
   The system will install/setup the LCD and reboot with the LCD as display.

To switch back to HDMI:

To use the HDMI port again, go to the LCD-show folder and run
sudo ./LCD-hdmi to switch back to HDMI.

Links

raspberrytips.nl circuitdigest.com github.com/goodtft/LCD-show