#Import modules
import arcpy
from arcpy import env

#Put file path for the folder containing mxds, where I have my sample file path
env.workspace = r"G:GeomaticsMap Requests2017-07-24_TrailClosures"

#Looping through all mxd documents, list layers and replace those layers MRGTrails layer with proper data source
for file in arcpy.ListFiles("*.mxd"):
    mxd = arcpy.mapping.MapDocument('files')
    df = arcpy.mapping.ListDataFrames("*.mxd", "Layers")[0]
    for lyr in arcpy.mapping.ListLayers(mxd, "*", df):
      if lyr.name == "Trails":
        lyr.replaceDataSource(r'G:GeomaticsDataRegionalMRG.gdb', "FILEGDB_WORKSPACE", "Trails_Sentiers", validate=False)

    mxd.save()

import arcpy
from arcpy import env

env.workspace = r"G:GeomaticsMap Requests2017-07-24_TrailClosures"

for file in arcpy.ListFiles("*.mxd"):
    mxd = arcpy.mapping.MapDocument(file) # Changed 'files' to file
    # Iterate over all data frames, because what if one has a different name...or if a map has multiple data frames
    # And pass the mxd object to the function--not the mxd wildcard
    for df in arcpy.mapping.ListDataFrames(mxd):
        for lyr in arcpy.mapping.ListLayers(mxd, "*", df):
            if lyr.name == "Trails":
                lyr.replaceDataSource(r'G:GeomaticsDataRegionalMRG.gdb', "FILEGDB_WORKSPACE", "Trails_Sentiers", validate=False)
    mxd.save()