The SIM card tray houses both the SIM card for your cellular network as well as the (optional) SD card for expanded storage. Use this guide to eject the tray and remove or replace either card. The S8 and S8+ both use the same tray, so the same part is compatible with either phone.
Insert a SIM card eject tool or a paperclip into the small hole in the SIM card tray, on the top edge of the phone.
Press firmly to eject the tray.
Grab the tray and pull straight out to remove it.