There is no 'button' for it. If you are trying to do a "Factory Reset", which gives you the option to try and save your data, then that is done within the phone's settings. However, if you are wanting to completely wipe the phone and start from scratch, that would be a "Hard Reset" and that is done as follows...
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3...