How do I reset TextExpander?

Just trash the Settings file. In v4 and earlier this file is called Settings.textexpander and in v5 it is called Settings.textexpandersettings.

TextExpander stores this file in your Library:

[Home]/Library/Application Support/TextExpander

(In OS X Lion and beyond (10.7 and later), the Library folder is hidden. In Finder's Go menu, hold down the Option key then select Library.)

However, if you are using Dropbox Sync, the settings are actually stored in your Dropbox, in the TextExpander folder.