You can't. You are able to customize some aspects of the button, but not the space between lines like you would do with a Word Document.
UPDATE: The only way to do this in the button of a form is with VBA. I tried some code like this:
Me.cmd_button.Caption = "LALALA " & vbNewLine & vbNewLine & "LALALALALALALA"
And i got this:
UPDATE 2: As @T.M. said, if you change the caption of the button, you must make sure to increase/reduce me.cmd_button.Height
property to make the text readable. If you use several breaklines and you do not change the height of the button, text won'tbe appear.