If you're not set up for automatic updating (I like to choose when and where I download all those bytes), you can just go Play Store > [Settings button] > My Apps, and it will show you any apps that have updates available. Then just click "Update 3" or whatever number it is, or click on each app individually to see what the update contains, and update the apps one by one.
For MX Mariner, you should also check in periodically to see when new charts
are available. Open the app, go to [Settings button] > Settings > Install Chart Imagery, then choose NOAA USA or whatever region you want, and you'll see which charts
have updates available. Charts cannot be set to update automatically (I think).