1. What's the make/model of the TV?
2. Are you talking about the typical user settings - if so, there is usually a "reset" somewhere in the picture mode or settings - it's not usually a button, but rather a menu item somewhere in the settings. Depending on its location this can reset the picture settings, or sometimes only a portion of the picture settings or sometimes the entire TV. This is usually outlined by the location of the reset - whether it's under the main TV settings, under the picture settings or a subset (like advanced) of the picture settings.
3. There may also be a reset in the main TV setup menu which would set the entire TV to the default settings.
4. Please see the following post, useful for those new to the forum:
HDTV Frequently Asked Questions: Please read this before posting in the forum