I've tried to play SP13 didn't work, said something about dreamseeker.exe and it said I should reinstall Byond, so I did, now it won't uninstall Byond, it says it open even though its not.
This is old, so replying to this is probably pointless, but here goes.

Open your task manager.

Search through the processes, clicking on any related to BYOND.exe, dreamseeker.exe, dreamdaemon.exe, and dreammaker.exe, and hitting "End Task."

Afterwords, try uninstalling. If that doesn't work, reply here.