Settings > Apps > Manage Apps > Three Dots (top right) > Default Apps > Phone and select the new MIUI icon.
Because it's not on the Play Store, getting the MIUI dialer requires some technical steps. The method you choose depends heavily on your phone's software version. miui dialer play store
Downloading APKs from unknown sources can expose your phone to malware. Xiaomi recommends obtaining apps exclusively from Xiaomi GetApps or Google Play, which have built-in security scanning mechanisms. Settings > Apps > Manage Apps > Three