reference an entity outside of the node/add or node/edit forms I will explain you with my use case:

I have a content type "job post" and another content type "Exam" For apply to a job you must pass the exam, the "job post" content type has an entity reference field "field_exam" that is referencing the "Exam".

While user creates the "job post" he can create his own "Exam" or he can Call for tender on creation of an "exam" for the "job post". In that case the User will recieve "exams" of other users and he will choose one of them for his "job post".

The list of options is a view of "exams", in front of the title of each "exam" should be a link that says "Choose this Exam", and if I click that link the field "field_exam" of the "job post" must be referencing that "exam".

So the main point of this is, how to change update the "field_exam" value selecting the entity outside, for example in a view or in the node page...

Thanks for your help.
You could use the Views and Rules Link modules (along with the Rules module) to accomplish this.

What you would do is create a Rules Link that will attach your line item (exam) to a Job (that the user selects).

You would likely need to have an  field available for Rules to manipulate, but there would be no forms involved directly using an Entity Reference field (no forms involved).