Hi
If you are using marshmallow (at least on a Samsung Galaxy phone)
settings
default apps
set as default
This lists all your apps and whether they are set as default. Select the apps concerned and clear or set the defaults. If you clear a default with setting a new default app, you will be given a choice next time you open a file, but your choice will become the default. If you dont like this behaviour there is an app "Better Open With" in Playstore which returns the choice each time a file is opened.
With older android versions you need to do this in
settings
manage apps
but older versions ask you each time which app you would like to use until you choose "alway use", after which the default must be cleared to get the choice again.