Digimon Story: Cyber Sleuth - Hacker's Memory

Accessories are bought from the Online Colisseum's shop using coins you win from Network Battles however if you import a save from Cyber Sleuth with "clear" on it when you started Hacker's Memory for the first time you will be given Cyber Sleuth Goggles as a reward.
Go into theDigimon menu from your Digivice and select the "settings" option. Choose the Digimon you want to equip and select the "accessories" tab at the bottom. Any unused accessories will be shown and you just need to select one. Accessories don't adjust stats or powers but you'll see what's equipped if your Digimon follow you on screen and when you enter the accessories menu.