Randalll Garrett & Robert Silverberg 
The Girl from Bodies, Inc. [EPUB ebook] 

In this gripping tale of switched identities, Hugh Horner’s simple wish for renewed youth thrusts him into a nightmare. A mysterious massage parlor promises the impossible: a chance to trade bodies with a younger man. But Horner soon finds himself trapped in the body of a convicted murderer, desperate to reclaim his old life. With the police closing in and time running out, Horner must outwit the devious figures behind the body-swapping scheme.