Please Follow the following steps
Step 1 :- Download the APK you want to install and make sure you have a file browser installed. Step 2 :- Navigate into your phone settings. Tap on the Security option. Step 3 :- Scroll down until you see Unknown sources and check the box. Step 1 :- Open your file browser and navigate to your download folder where the APK is. Tap the APK and follow the instructions to install the app. Step 1 :- Once done, return to your settings and untick the Unknown sources box to prevent any additional unwanted installations.