Replace data frame with arcpy

I work on arcview 10.3. How can i replace an existing data frame in mxd with another data frame by using arcpy? I started write this code but i don't know how to do it:

import arcpy,os,sys,string
import arcpy.mapping
from arcpy import env

mxd1 = arcpy.mapping.MapDocument(r"C:\Project\Project.mxd")
mxd2 = arcpy.mapping.MapDocument(r"C:\Project\Project2.mxd")
df1 = arcpy.mapping.ListDataFrames(mxd, "Layers")[0]
df2 = arcpy.mapping.ListDataFrames(mxd, "Layers")[0]

mxd.save()
del  mxd