How to add my ex wife's name to a house title?

A quitclaim deed would do it.

But why would you want to?

Because we have a mutual agreement and I think that's far since we both owned that house at some point and her name is still on mortgage. She removed her name 6 y ago but we both agreed to add her back again. Things are different now between us. Also, do we need to pay any taxes if we do add her name.
Thank you for your help.

... Also, do we need to pay any taxes if we do add her name. ...

In theory, yes. The interest you transfer to her may be subject to the gift tax (http://en.wikipedia.org/wiki/Gift_tax_in_the_United_States), if it is over $13,000 in value.

At the top of the Real Estate Law forum there is a sticky post that explains how to add someone to a deed. You can read it at https://www.askmehelpdesk.com/real-estate-law/how-add-someone-your-deed-569716.html

I am understanding it correctly; there is a Federal Estate Tax Exemption up to $5,000,000? That means if I add her name I or her would not have to pay tax. Once again thatnk you for your help.

BTW, not sure if this is important; my ex wife is the one who currently lives in house and her name is still on mortgage (both us of are on mortgage).

I am understanding it correctly; there is a Federal Estate Tax Exemption up to $5,000,000? That means if I add her name I or her would not have to pay tax. Once again thatnk you for your help.

No. That's the lifetime gift exemption. If you make $10,000 gifts to 500 people this year, next year your lifetime gift exemption will be all used up. Any gifts you make next year will be taxable.

House is worth $400,000.00 so I guess that qualifies under the Lifetime gift exemption?