First, access your Pipe’s Settings by selecting the settings wheel on the upper right side of your Pipe Dashboard. Then, inside your Pipe’s Settings, you can either choose to edit your Start Form (select the 'Start Form'-tab) or a Phase (select the 'Phases'-tab).
Inside the Field’s settings, scroll down and click the red 'Delete field'-button. Take a moment to consider it, once you’ve deleted a Field, all the information in it will be deleted as well.
Pipefy Tip: There's a shortcut to deleting Fields! Click the three little dots on the right side of your Field's title and click 'Delete Field'