Why do I receive attribute error for replacing a cells content in a python dataframe?

I tried

df3.at[294,'Initial Effective Date'].str.replace("5/26/19 and 8/2/20","8/2/20")

My dataframe(df3) has multiple columns and 300 rows of data. I have a column called initial eff date
I just want to replace a cell at row 294 as it as above. but I get an error saying
attribute error: ‘str’ object has no attribute ‘str’

>Solution :

Try

df3.at[294,'Initial Effective Date'] = "8/2/20"

.str.replace() method is for Series and you get only one cell (String) with .at()