The only way I recall is to buy the SD cards with updated maps from the dealer or a supplier. You can't make pirated copies too easily, or at least last time I checked. I think they stopped updates for that unit years ago, so I'm not sure how you'd obtain anything more recent than 2015 changes or whatever.
To be honest, the 315 is a horrible unit for both audio and nav quality and consistency. Often I find even projects completed in 2010 aren't on my 2011, and changes to roads/streets etc. Better to replace the HU with something OEM-like that uses your phone's google maps/nav or has its own antenna. Just my $0.02.