I have by mistaken clicked on the upgrade button even though I have 300$ free credit. Will GCP charged me?

Nehasharma,

You should be fine and won’t be charged unless you go beyond the limits of the free tier. When you upgrade prior to the end of your trial the $300 credit is just applied towards your upgraded account.  There are more details here:

https://cloud.google.com/free/docs/gcp-free-tier#how-to-upgrade