Convert dataframe from str back to datafarme

[Creepy]

Hi,

Am looking for help where I converted a dataframe in order for replace function to work?
Now am looking for way to convert it back to datafame ( post the replace) for further processing.

df = str(df)
    df = df.replace('old_value','new_value')

Without converting it to str .replace doesn't work on my dataframe.