Default Applications Configuration

Using System Settings

In the System Settings screen, go to Personalization > Applications > Default Applications, then use the menus to select your preferred default applications.

Using Dolphin

Open Dolphin.
Navigate to the folder that contains the file you want to open.
Right-click the file.
Go to Open With > Other Application.

The Choose Application dialog appears.
Choose the application you want to use from the list.
Select Remember application association for all files of type [file type].
Click OK.