您可以在整列中使用str.replace:

from io import StringIO
import pandas as pd

data = """sku,name,code  
k1,aaa,886  
k2,bbb,898  
k3,ccc,342  
k4,ddd,503  
k5,eee,401"""

file = StringIO(data)

df = pd.read_csv(file)
df['sku'] = df['sku'].str.replace('k', '_')

print(df)

这产生

  sku name  code  
0  _1  aaa     886
1  _2  bbb     898
2  _3  ccc     342
3  _4  ddd     503
4  _5  eee     401