I want to write a function to help me add/update or delete a custom property in my org-mode's entries. But I'm not really sure where to start, are there org functions that can help me manipulating this data or I have to parse an entry by myself?
org-entry-put does this:
org-entry-put is a compiled Lisp function in `org.el'. (org-entry-put POM PROPERTY VALUE) Set PROPERTY to VALUE for entry at point-or-marker POM. If the value is `nil', it is converted to the empty string. If it is not a string, an error is raised.
You'll have to either move point to the position of the entry you want to modify, or somehow get its point.
For more information on the POM argument, you may want to read this other answer.