Hi Guys !

So, I just finished my e-filing for individual tax and due to sheer stupid overconfidence, I computed my total income based on gross data from my salary slips that I have on file, and not based on the company issued EA form as prescribed.

The issue arises when, due to carelessness, I accidentally left a full months pay out of my total taxable salary.
I then signed and submitted the form thinking that I will soon receive a nice refund from the government, only to realize my error later.

How would I go about rectifying this problem ?
Also, what would the penalties be for under-declaring your income by a months' worth ?

Thank you very much in advance for your help !