You can edit any OER and even remix together several OERs to obtain a  perfectly customized result. There are, however, several things to keep in mind:

No two textbooks are quite the same - so if you are using multiple OER to create a custom textbook, you will need to blend together and harmonize the units and their learning objectives, as well as deciding on the specific pedagogical elements you want, and ensuring that your final text is consistent in that area.

Openers

● overviews (previews)
● introductions
● outlines (text, bullets or graphics)
● focus questions (knowledge and comprehension questions)
● learning goals / objectives / outcomes / competences / skills
● case problem
● In addition one may use the "special features" used inside chapters, e.g. vignettes, photos, quotations...

Closers

● conclusions and summaries (may include diagrams)
● list of definitions
● reference boxes (e.g. computer instructions)
● review questions
● self-assessment (usually simple quizzes)
● small exercises
● substantial exercises and problem cases
● fill-in tables (for "learning-in-action" books) to prepare a real world task
● ideas for projects (academic or real world)
● bibliographies and links (that can be annotated)

Integrated Pedagogical Devices

● emphasis (bold face) of words
● marginalia that summarize paragraphs
● lists that highlight main points
● summary tables and graphics
● cross-references that link backwards (or sometimes forwards) to important concepts
● markers to identify embedded subjects (e.g. an "external" term used and that needs explanation)
● study and review questions
● pedagogical illustrations (concepts rendered graphically)
● tips (to insure that the learner doesn't get caught in misconceptions or procedural errors)
● reminders (e.g. make sure that something that was previously introduced is remembered)
● case studies
● problem descriptions
● debates and reflections
● profiles (case descriptions)
● primary sources and data
● models

The list above is from "Textbook writing tutorial" by Daniel K. Schneider licensed under CC BY-NC-SA 4.0.

You will have to harmonize different writing styes, including POV (first person? third person?), vocabulary, and the over-all writing/reading level.

An example would be if you liked a concept you saw in a graduate-level open access scholarly book, and wanted to add it to a textbook for a 300 level class. You might have re-write it a bit, possibly simplifying the style, to make it more easily readable at your students' level.

If making your OER as accessible as is reasonably possible  or Universal Design for Learning is important to you, you need to assess the state of the OER materials you are considering adopting or revising - remediation may be necessary.

Similarly, an otherwise decent OER may not be as equitable or inclusive as you want it to be - will today's increasingly diverse students be able to see themselves (literally, such as in relevant images, or more figuratively, including names and descriptions) within the text? Assessment and remediation may be necessary here, as well.

All openly-licensed material used within an OER work of yours must be correctly attributed. Attributions are best added right below the openly-licensed material.

Not all CC Open Licenses used for OERs are compatible. Each CC open license is a legal tool, representing terms of use under copyright - and some licenses' terms simply conflict with those of other CC licenses.

An easy way to see which licenses are compatible with others (and which ones aren't) is to compare licenses on the following chart:

If you do any substantial amount of revising, editing and remixing, you'll have created a new work - and will need to pick which CC license you want to use. As you do so, be mindful of the possibility that your chosen CC License may clash with OER materials already within your work (see the chart just above).

For help with revising, remixing, and attributing and licensing correctly, contact the Henry Whittemore Library OER Liaison, Rebecca Dowgiert at rdowgiert@framingham.edu.