To edit your Items within PaperUI you need to:
PaperUI->Configuration->System->Item Linking->Turn off “Simple Mode”
You will be then presented with a new menu on the left side (just below the one that you have selected now (Things).
You can then edit the Items and add Labels
For Tags & REST API, see here a short guide: Official Alexa Smart Home Skill for openHAB 2
Check out also: HowTo: Listen & Talk to your Home
If you are an advanced user, you may want to consider using flat text config files for your *.items