You should wait until your return has been accepted or rejected. If rejected, add the W-2 before you re-submit. If accepted, you need to wait until your refund has been received or your taxes due cleared your bank account. 

It's important to wait because sometimes the IRS can make adjustments to your return automatically (and then you wouldn't need to amend).  Once you've refund or payment has cleared your account, you can come back into your TurboTax account and choose to amend your return.

At this moment, it's too early to amend. Once amended returns are ready, you'll find the instructions by clicking 2017 at the following link below: https://ttlc.intuit.com/replies/3288565