Satya,
It depends on how you configured your certification campaign template.
If the execution step, initiate approvals, the chekbox of "Request approvals before implementing changes" is checked, there are two options:
a) If the radio button of "As each certifier submits changes" is selected, then when a certifier clicks submit on his review, it is processed immediately.
b) If the radio button of "After all certifiers complete their reviews", then the reveiwers decisions can be changed.
The decisions can be changed in the Administration/Workflows/relevant campaign/progress by reviewers.
Click on the relevant reviewer and then on the relevant user.
Now you can modify your decisions.
this last option is the only option I know of that you can modify the reviewer's decisions.
Please let me know if this answers your question.
Kind regards,
Nino Uziel