Xiaomi
Change Default Messaging App auf Xiaomi
On Xiaomi phones running MIUI, the messaging app that opens for texts and notifications is controlled by a separate default app setting, not by whichever app you used last. Installing a new SMS app from the Play Store does not switch this assignment automatically. That is why replies, quick actions, and notification taps can keep opening an app you no longer want to use.
To change it, go to Tap on Settings ➔ Tap on Apps ➔ Tap on Manage apps ➔ Open the Menu ➔ Tap on Default apps ➔ Tap on Messaging ➔ Set the default Messaging app. Selecting an app here makes it the one that handles incoming texts, notification taps, and any share-to-message action from other apps.
If the app you want to set is missing from the list, install it first and open it once so MIUI registers it as a valid messaging app. If texts still open the wrong app afterward, check whether a second messaging app is running in the background and clear its notification permissions, since some apps re-register themselves as default on launch.
This setting only affects which app opens for messaging actions. It does not delete messages, move contacts, or change SIM settings, so switching back later is safe and reversible.