Koraslide stores the name, picture and access token of each Facebook Page or Instagram account you connect, so it can publish the posts you schedule. Nothing else from Facebook or Instagram is kept.
In Koraslide, open Connections and disconnect Facebook and Instagram. Their tokens are deleted immediately.
Or, on Facebook, go to Settings → Apps and websites, remove Koraslide, and choose to send a deletion request. Koraslide deletes everything it holds from those connections as soon as Facebook tells it to.
To delete your whole Koraslide account, contact support from the email you signed up with.