Instruction on how to revise a manual

I am not aware of any standard covering requirements concerning specifically about revising a quality manual. With ISO 9001:2015 a quality manual is no longer mandatory. So, you may apply the general requirements applicable to any document.

You can find more information about documentation below:

• New approach to document and record control in ISO 9001:2015 - https://advisera.com/9001academy/blog/2015/06/30/new-approach-to-document-and-record-control-in-iso-90012015/
• Some tips to make Document Control more useful for your QMS - https://advisera.com/9001academy/blog/2014/05/20/tips-make-document-control-useful-qms/
• Free online training ISO 9001:2015 Foundations Course - https://advisera.com/training/iso-9001-foundations-course/
• Book - Managing ISO Documentation: A Plain English Guide - https://advisera.com/books/managing-iso-documentation-plain-english-guide/
   

what documentation then is required if a quality manual is not required?

In this article - List of mandatory documents required by ISO 9001:2015 – https://advisera.com/9001academy/knowledgebase/list-of-mandatory-documents-required-by-iso-90012015/ - you can see the mandatory documents and records according to ISO 9001:2015