import pandas as pd

bad_px = ["12", "34"]

df = pd.DataFrame({
    "px_id": [1, 2, 3],
    "px": ["12", "34", "56"],
    "px_type": ["10", "10", "09"]
})

print("Original: \n{}".format(df))
index = df["px"].isin(bad_px)
df.ix[index, "px_type"] = '09'
print("Fixed: \n {} ".format(df))

# Original:
#    px  px_id px_type
# 0  12      1      10
# 1  34      2      10
# 2  56      3      09
# Fixed:
#     px  px_id px_type
# 0  12      1      09
# 1  34      2      09
# 2  56      3      09