方法一：使用到select

以下面的将Names列的名字中的每个单词首字母改为大写字母为栗子：

spark = SparkSession.builder.appName('SparkByExamples.com').getOrCreate()

columns = ["Seqno","Name"]
data = [("1", "john jones"),
    ("2", "tracey smith"),
    ("3", "amy sanders")]

df = spark.createDataFrame(data=data,schema=columns)
df.show(truncate=False)

+-----+------------+
|Seqno|Names       |
+-----+------------+
|1    |john jones  |
|2    |tracey smith|
|3    |amy sanders |
+-----+------------+

def convertCase(str):
    resStr=""
    arr = str.split(" ")
    for x in arr:
       resStr= resStr + x[0:1].upper() + x[1:len(x)] + " "
    return resStr 


""" 将函数转为udf """
convertUDF = udf(lambda z: convertCase(z),StringType())

""" 默认返回值是 StringType()，所以上面不执行也行 """
convertUDF = udf(lambda z: convertCase(z)) 


df.select(col("Seqno"), \
    convertUDF(col("Name")).alias("Name") ) \
   .show(truncate=False)

+-----+-------------+
|Seqno|Name         |
+-----+-------------+
|1    |John Jones   |
|2    |Tracey Smith |
|3    |Amy Sanders  |
+-----+-------------+

方法二：使用withColumn

def upperCase(str):
    return str.upper()

upperCaseUDF = udf(lambda z:upperCase(z),StringType())   

df.withColumn("Cureated Name", upperCaseUDF(col("Name"))) \
  .show(truncate=False)


+-----+------------+-------------+
|Seqno|Name        |Cureated Name|
+-----+------------+-------------+
|1    |john jones  |JOHN JONES   |
|2    |tracey smith|TRACEY SMITH |
|3    |amy sanders |AMY SANDERS  |
+-----+------------+-------------+

Reference

[1] https://sparkbyexamples.com/pyspark/pyspark-udf-user-defined-function/