Verify that what you want to report is in fact an error. Even relatively straightforward information, such as dates of birth or spelling of place names, are often open to dispute, and the textbook may be reporting something that's accepted as true by many reputable scholars. Research the topic thoroughly to see if there are grounds to defend the current information in the textbook.
Collect documentation that supports the proposed correction. Appropriate documentation includes published data in scholarly sources.
Identify the publisher's and author's contact information. The publisher's information will be included in the front matter, usually immediately after the title page. The author's contact information may not be as easy to obtain, but it will probably be on the back flap of the dust jacket or at the end of the introduction. If it's a scholarly textbook, the author will probably list an academic affiliation, and you can visit the college website to get contact information.
Send your correction and supporting documentation to the publisher and author. If you are reporting typographical errors only, you may choose to report them to the publisher alone, without involving the author.