git proper way to fix topic after branch merge

I'm new to Git and for now I'm only working with a local repository but I will eventually share my work with other developers. I'm trying to figure out what is the proper way of fixing code on a topic branch after it's been merged back to the master branch. Here's what I have:

My topic was implemented in C2 in a topic branch and then merged on the master branch. There's been another commit C5 on master for a different topic. Now I just found an issue in with the feature implemented in C2. Should I fix it in the topic branch as C6 then merge again on the master branch? Or should I just get rid of the topic branch since it's been already merged and just fix it directly in the master branch?

Ideally I would like to keep the C2 and C6 in a single commit so that when we implement remote repository I can push a clean history. But I don't think I can just squash C2 and C6 because of the merge. Is there a way to do this?

What would be considered good practice to fix a topic after it's been merged?

It will be helpful for you to learn about git rebase for managing your local history. For example, consider this history of commands, which I think roughly recreates your situation and and a possible resolution using git rebase -i to rearrange your local history:

mkdir tmpfoo
cd tmpfoo/
git init
touch bar
git add .
git commit -am 'c1'
git checkout -b topic
touch baz
git add .
git commit -am 'c2'
git checkout master
touch foo 
git add .
git commit -am 'c3'
git merge topic
touch quux
git add .
git commit -am 'c5'
git checkout topic 
echo b >> baz 
git commit -am 'c6'
git checkout master
git merge topic
git log --oneline --graph --decorate --all
*   577f974 (HEAD, master) Merge branch 'topic'
|\
| * 6750b0d (topic) c6
* | 08ebbf2 c5
* |   0cef647 Merge branch 'topic'
|\ \
| |/
| * f1e6882 c2
* | 0e19228 c3
|/
* 90e6149 c1
19933  git rebase -i 90e6149
>>edit the sequence of commits to something like the following:
pick 0e19228 c3
pick 08ebbf2 c5
pick f1e6882 c2
s 6750b0d c6
19934  git log --oneline --graph --decorate --all
* 0e3afe0 (HEAD, master) c2
* 7611fb9 c5
* 0e19228 c3
| * 6750b0d (topic) c6
| * f1e6882 c2
|/
* 90e6149 c1
git diff ORIG_HEAD # no output here, the rebase didn't change the working tree's state