When you have two or more applications that do the same thing, like browsers, Android prompts you to choose one to them as your default app to open a specific file type, until you do the same.
Changing the default app with a new one was a tricky process in past but the things have changed after the arrival of Android Marshmallow 6.0 and Nougat 7.0.
The release of last two updated of Android doesn’t mean that they will be available for your device. Their availability dependents on your device’s manufacturer, model, and the carrier.
To see which version of software is installed on your device, open home screen of your device and tap on Apps. Then go to Settings > General > About Phone or About Tablet and click on Software information.
Follow the steps below to change your default apps if you have updated your device to Marshmallow or Nougat. The wordings and their locations vary in different devices. Also See How to Update an Android Phone
Go to the home screen of your device, swipe down the notification panel and click the cog icon to enter Settings.
Now look for Apps section. It is named as Apps or Manage Apps in most devices but on Samsung Galaxy devices you’ll need to find Applications. The users of LG handsets can find Apps option under General tab.
If you are running Marshmallow, tap the cog at the top upper-right and then select Default Apps (or Default Applications if you are a Galaxy user). The Default Apps options is not available in Nougat and you can manage it from App Settings section.
If you are an LG user, navigate to the top right corner, tap on overflow button and select Configure Apps.
You will be now redirected to a new page where you will see everything you have listed as default on your device. Here you can change browser, launcher, dialer, SMS apps and other elements.
For example, if you want to change the default browser, tap on browser app and you will be presented a list of apps that can open web links. Select the one you want to set as a default.
It is important to mention that here that every time you’ll install an app that can be set as a default, your preferences for that category will be reset. This will make easier for you to set the newly-installed app as a default.