I'll admit I haven't taken great care of the wife's car, so don't get on me about what I should have done. It's a 2015 Tahoe 5.3L six speed with 167k miles. The transmission fluid has not been changed that I'm aware of (bought it used). The fluid is not a pretty pink, and doesn't smell burnt, however it obviously needs to be changed. My question is if I changed the fluid will it help it stop slipping or do you think the transmission will need to be replaced or rebuilt?

Thank you in advance for your help,
Louie

 

If it's slipping right now, then the transmission is done. Lack of maintenance has done it in.
Changing the fluid at this point will not magically reconstruct the clutches, and there's a chance it will stop shifting completely.
It needs to be replaced with a remanufactured unit. Never rebuild.