Make sure that there should not be any prior version of Arm Wrestling Clicker MOD APK apk already installed on your phone. If so, uninstall that before installing this one.

First of all, download the Arm Wrestling Clicker MOD APK APK from the above download link
Install the Split APK installer app and then install the downloaded mod APK using that. (or) follow the below instructions
Enable app install from Unknown Sources to install apps from a third-party app other than Google Play Store.
To do that go to Settings and search the word unknown- For Android Nougat or lower version click on Unknown sources and enable that setting.
- For Android Oreo or later, clicking on Install unknown apps which will take you to a list of apps.
Select the actual app through which you want to install the apk of Arm Wrestling Clicker MOD APK.Or you can do it other way: Let's say, you have downloaded the APK file of Arm Wrestling Clicker MOD APK on Google chrome or firefox browser on your phone.Go to Settings > Apps & notifications or go to Settings > search for unknown > Open the app (Chrome, Firefox or file manager app etc) > Install unknown apps and enable Allow from this source
Once enabled, tap on the hack (or) mod apk file to install