what if by mistake we click on upgrade button ? how to undo this? Is it possible to downgrade to account to free trail again?
You may contact Billing Support, but I think that the process is not reversible (unless the Support of GCP agree you help you doing that, somehow).
Although you’re no longer in free trial tier, your free credit balance remains and you can continue using it. Any expense will be deducted from this free credit balance. Only when your consumption is higher than your free credit balance, your actual credit card will be charged.
GCP has "always-free" tier services, so as long as you deploy your lab within those limits, you won’t be charged. Check out the docs here.
You may want to disable billing from your account, so that you won’t be charged either (of course you won’t be able to use your free credit balance too, if there’s anything left).