You probably have both the Google Play and Xiaomi Market apps on your Xiaomi devices. Whenever you perform an action that requires to download an app, you’ll get the “Complete action using” prompt to ask you for a selection.
For example, when I tap the Candy Crush invite on Facebook (I know you hate it), I’ll be prompted to choose either Google Play or Xiaomi Market to download the Candy Crush app. I can select Google Play since I prefer to get the app from there. And to avoid getting similar prompt when I perform the same action again in the future, I can tick the Remember this choice option. By doing so, I’m actually setting Google Play as the default app for this action.
I hope you’re clear so far.
Now, what if you’ve picked the wrong app as the default ? No worries, you can always “clear default” and make a selection again.
So let’s say, I accidentally chose Xiaomi Market as the default market app for my example, I would need to get to the App info screen of Xiaomi market and then tap Clear defaults. Once I do that, I’ll see the “Complete action using” prompt, the next time I tap on the Candy Crush invite. And, there’s no reason for me to pick the wrong one this time !