Try

df = df.withColumnRenamed("colName", "newColName")\
       .withColumnRenamed("colName2", "newColName2")

Use 

new_column_name_list= list(map(lambda x: x.replace(" ", "_"), df.columns))

df = df.toDF(*new_column_name_list)

Use 

df.withColumnRenamed('age', 'age2')

Try

df = df.select( '*', F.col('count').alias('new_count') ).drop('count')