Can i mend my taxes and add my Grandson that was born before Jan.31 2017?

Yes if he can be your dependent.

You should *wait* until your return has been processed and you receive your refund or conformation that any tax due has been paid. (If you file an amended return while you first return is being processed it can cause extended delays for both returns if two returns are in the system at the same time). In addition, if the IRS makes any change on your original return, you might end up having to amend the amendment – a sticky process that can take a year or more).

-- Then you can start the amend process.

-- Amended returns can only be mailed - allow 8-12 weeks - can take up to 16 weeks (4 months) for processing.

See this TurboTax FAQ for help with amending:

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

You can check the status of your mailed amended return at this link but allow 3 weeks after mailing before checking.
http://www.irs.gov/Filing/Individuals/Amended-Returns-%28Form-1040-X%29/Wheres-My-Amended-Return-1

https://ttlc.intuit.com/replies/5114064

https://ttlc.intuit.com/replies/3288565