How to respond after co-authors revise rejected paper without informing first author? As we all know, it is common to re-work a rejected paper to improve the shortfalls of the paper.

Suppose one is first author on a paper, and it is rejected. Following this,


the other co-authors make some modifications/updates to the paper, and don't inform the first author
one co-author claims first authorship on the revised work, as he/she has done more work after the rejection


How should the first author respond after finding out about this?
A lot depends on the particulars of the situation, but the principles of academic honesty are pretty straightforward here, and an answer for the particular situation can be derived from those principles.

The basic principle is that every involved author in a work has a stake in that work and the consent of all authors must be obtained before a work is submitted.  Most journals actually explicitly ask about this, and the better run ones will send email to every author to inform them so that they have a chance to object.

Somebody who was once author, however, can become uninvolved in one of several ways:


The author may object to the nature of the paper and explicitly withdraw from authorship.
The author may stop fulfilling their responsibilities for authors, in which case they should be kept as an author but might have their position in the author list change based on how the manuscript continues to evolve. 
The author may have had a minor role (e.g., in a many-author biology or physics publication), and choose to give blanket consent to whatever the primary authors believe is acceptable, rather than be involved in detail.


So the first thing to determine is, did the first author effectively drop out of their responsibilities?  As the lead author, a first author needs to discuss the fate of the rejected paper with the other authors in a timely fashion.  If the first author instead vanishes (e.g., changing positions and not staying in communication about the work), then it is entirely reasonable for the others to pick up the paper again.  The first author might or might not stay first then, depending on how the paper evolves, how much new experimental work is done, etc.  If there is a decision not to proceed, however, then anyone who changes their mind and wants to revise and resubmit needs to re-involve all of the authors again.

If it is the case that the others acted inappropriately, then the action to take depends on how important the paper is to the first author relative to the relationship with the other authors.  The range of options includes (from least to most bridge-burning):


Ask to be involved in further revision and authorship discussions (assuming the paper isn't already published)
Stop collaborating with the other authors (if the first author is aiming to be a career scientist, there will be many other papers with better collaborators)
Request a correction or retraction from the publisher


Scientists are people like everybody else, and have everything from stellar working relationships to innocent miscommunications to nasty misbehaviors.  Start from the assumption that there is an innocent misunderstanding, but if there is real misconduct involved, the best thing to do is to balance protection of yourself and warning of others as best fits your situation.