Yes, There is no option to change Income once retun is filed. You have to file the Revised return to change your income. It's not recommendary to revised the return except it's really necessary.
If client want to show more income than you can file revise return and show the increased income in Revised ITR. You can file revised ITR by selecting return under section 139(5) and also mentioning acknowledgement number of original ITR filed with your correct income (increased income).
Yes, Not mandatorily, ...but there is a change of notice. That's why I said that " It's not recommendary to revised the return except it's really necessary"