The first thing that many people do when confronted by a pre-written program, whether in book form or written for them by a trainer or found on the web, is to ask, "Can I change the program?"
No. But also, yes.
No, you can't.
It isn't the same program if you change it. If the program is really the right program for your goals and situation, changing it will reduce its effectiveness. The more precisely the program matches your needs, the more changing it is counterproductive.
You might lack the knowledge of the program maker. This seems harsh, but it is often true. If you seeking to take advantage of someone else's knowledge of programming and training, it is probably because you need some kind of guidance in training. You might even know a lot about training, but need an outside perspective on what approach you need. They may have specific experience in doing a specific type of training - powerlifting, strongman, endurance, marathon running, etc. If you need to seek outside help in this specific area, it makes sense to take the advice in whole instead of in part.
Yes, you can.
Of course you can.
It's your workout. You have to do the lifts, run the miles, and take the rest days. You know yourself, and a generic program just makes assumptions about the trainee. If you can't do a specific exercise and have to make a substitution, of course you can do it.
Nothing is truly graven in stone. Even the person who wrote the program would make changes for trainees who have different needs and different levels of ability. Changes might change the effectiveness of the program but it can and is done.
An Important Caveat: If you have to ask, you probably shouldn't change the program. If you know enough about your training to know what can be changed and what cannot, you know enough that you don't need to ask.
However, if you change it, you own it. It is no longer "Starting Strength" or "Westside" or "NROL," it is your program. It's success or failure is your success or failure, not that of the program. Accept this, and own this.