Set the time of datetime column with condition

I have a dataframe that it has a datetime column ‘started_at’

I would like to change the time to it, if the time is smaller that a specific time (12:00)

So, the data should become:

Any suggestions?

>Solution :

Maybe you can try this

import datetime as dt
for i in range(len(df)):
    fmt = '%Y-%m-%d %H:%M:%S'
    val = dt.datetime.strptime(str(df.loc[i][0]), fmt )
    new_val = dt.datetime.strptime(str(df.loc[i][0]).split(" ")[0] + " 12:00:00", fmt )
    if val < new_val:
        df["started_at"] = df["started_at"].replace(val,new_val)