Change EPSG code of a geojson file created manually

Hi all
I created the geoJson file but when I run the following commands to know the EPSG number of the file:

from osgeo import ogr, osr
import os
driver = ogr.GetDriverByName('GeoJSON')
inDataSet = driver.Open("./EPSG3857.json")
inLayer = inDataSet.GetLayer()
inSpatialRef = inLayer.GetSpatialRef()
print("inSpatialRef ", inSpatialRef)
sr = osr.SpatialReference(str(inSpatialRef))
res = sr.AutoIdentifyEPSG()
srid = sr.GetAuthorityCode(None)

I obtain EPSG:4326 while the coordinates are in EPSG:3857. It is possible to change EPSG system for json file? I tryed with SetAuth