Take a look at the Park Tool Repair blog that offers instructions on how to replace these cables: https://www.parktool.com/blog/repair-help/nuvinci-n360-hub-installation-and-adjustment
Also - here’s a video from Enviolo that highlights the procedure: https://support.enviolo.com/hc/en-us/articles/11954208675858-How-to-replace-shifter-cables-#video-instructions
The shift cables are standard bicycle derailleur cables, available online or at any local bicycle shop.