What should one who took money entrusted to him for delivery, then regretted it, do?

As long as the donor has specified a particular recipient for her charity, it is not permissible for you to take from it, even if you are in need. You must repent to Allah, and you are responsible for the money; you must deliver it to the recipient specified by the donor, or seek forgiveness from the donor and ask her to absolve you.