change pandas column value based on condition

In [41]:
df.loc[df['First Season'] > 1990, 'First Season'] = 1
df

Out[41]:
                 Team  First Season  Total Games
0      Dallas Cowboys          1960          894
1       Chicago Bears          1920         1357
2   Green Bay Packers          1921         1339
3      Miami Dolphins          1966          792
4    Baltimore Ravens             1          326
5  San Franciso 49ers          1950         1003

Here is what the above code is Doing:
1. We’re using the loc[] method to select all rows where the value in the ‘First Season’ column is greater than 1990.
2. We’re setting the value in those rows to 1.
3. We’re printing out the entire DataFrame to see the results.