“Who” and “whom” are relative pronouns for persons. They begin relative clauses by replacing or referring to persons or animals with names either subjectively (you need to know “who” is responsible) or objectively (you need to know “whom” to contact).
The word “who” substitutes for the subjective-case pronouns “he,” “she” or “they”; “whom” substitutes for the objective-case pronouns “him,” “her” or “them.” If you don’t want to use “whom,” restructure your sentence. Don’t just use “who” when it is incorrect.