Unbanning someone on Discord is a straightforward process, provided you have the necessary permissions for how to unban someone on discord:

Server Settings: Go to your Discord server and click on the server name at the top left.
Server Settings: In the drop-down menu, select "Server Settings."
Moderation: In the left sidebar, choose "Moderation."
Bans: You'll see a list of banned users. Find the user you want to unban and click on their name.
Unban: A pop-up window will appear; click the "Revoke Ban" button.
Confirmation: Confirm the action when prompted. The user will be unbanned and can rejoin the server.