Connect your Switch microSD card to your PC or use a USB transfer tool like DBI or Tinfoil .
Ensure your custom firmware has the latest Sigpatches installed, or the game may fail to launch with a "corrupt data" error after the update. GTAVC-(USA)-NSwTcH-NSP-Update107-Ziperto.part1.rar
To install the update on your Switch (which must be running custom firmware): Connect your Switch microSD card to your PC