Board Thread:Questions and Answers/@comment-27689581-20160127231653/@comment-26521006-20160128000952

Depends, I believe the steam version of Undertale backs up your files. I'd suggest manually checking and deleting everything in the folder that your saves are saved in just in case. Save location(s) are:
 * Windows XP:
 * %SystemDrive%/Documents and Settings/%USERNAME%/Local/Application Data/UNDERTALE and %SystemDrive%/Documents
 * %SystemDrive%/Documents and Settings/%USERNAME%/Application Data/Steam/CODEX/280360/remote/
 * Windows Vista/7/8/10:
 * %SystemDrive%/Users/%USERNAME%/AppData/Local/UNDERTALE
 * %SystemDrive%/Users/%USERNAME%/AppData/Roaming/Steam/CODEX/280360/remote/
 * Mac: ☀~/Library/Application Support/com.tobyfox.undertale/