While many modern apps (like WhatsApp) no longer work on Windows Phone 8.1, you can still find legacy versions of useful software on sites like Mobyware :

Find application files (in .xap or .appx format) on community forums like 4PDA or dedicated archives.

Use a PC tool like or CMD Injector to push the files to your device.

: You can download .xap files to a computer, move them to a memory card, and then select "Install local apps" from the three-dot menu in the phone's Store app (though this may fail if the app requires online validation). Popular Apps for Lumia 630