I sent in my tax return via mail but forgot to include my W2. What should I do?

If your return has been paper-filed , you cannot change the information. Instead, you have to amend a return.  See instructions below.

IRS does not allow e-file for amendment,  you will need to submit the amendment forms with related supporting documents by mail. (Follow instructions through the step-by-step interview questions in the program)

If your original return generates a refund, you should wait until your original refund arrives before you file your amendment. You can cash your check, but you still should wait until your current tax return is resolved.  Note that if you see " Do Not File" watermark on your Form 1040-X, this means your original return is still pending.

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

[Edited 2/6/2017]