BIG launcher is available for download in the Google Play Store. The best way is to search for it. You can choose between free demo and full version. You have to use your credit card to buy the full version. When installed, press the open button to run it. If you want to use a different language, choose it from the menu. Then choose your desired text size and color theme. When you press the home button for the first time, you will be offered to choose from the installed launchers. Check 'Use by default' and choose BIG Launcher from the menu. Now, whenever you press the home button, you'll get back to the home screen of BIG Launcher.
If this doesn't work, you have to unset your default launcher manually. Go to the system settings. Choose 'applications'. And switch to the section where you can see all the apps. Now you have to find your current launcher. It is different on every phone. When you find it, tap it and then tap the 'clear defaults' button. now, when you press the home button, you can check 'use by default' and choose the BIG Launcher from the menu.