Using Python how can I merge two columns and overwrite data from one column only if data in other column exists?

I am trying to merge some data and have been unable to get the results I've been looking for. I have two data frames with two columns each: CID and Notional. DF1 has every CID and DF2 has just some of the CIDs. I want to take DF2's data and merge it with DF1 so that IF DF2's data exists it will overwrite DF1, and if not DF1 will retain it's data.

I have tried using pd.merge and I end up with a DataFrame that has columns CID, Notional_X, Notional_Y; I have tried 'update' but it just replaces all old DataFrame data.

Here's an example of what I'm looking for:

#Example of Data (couldn't find a better way to explain this)
df1 = pd.DataFrame({'CID':[1,25,100], 'Notional': [1000, 2500, 5500]})
df2 = pd.DataFrame({'CID':[25], 'Notional': [0]})

the output would return a DataFrame that looks like this:

pd.DataFrame({'CID': [1,25,100], 'Notional': [1000,0,5500]})

(not that the merge reduced CID 25 to 0 which is found in df2 without changing anything else)

The documentation suggests that 'merge' should accomplish it but it just... doesn't.

test = df1.merge(df1, df2, how = 'left', on = 'CID')

This seems to merge the dataframes without merging the data (it just appends a column on the end)

Any help would be greatly appreciated. Thank you.