How can I return the excess amounts that were unduly paid to me by my employer, and should that be through charity or by returning them hand-to-hand?

You may accept the increase from your employers if they give it willingly and without any lying or deception on your part. It is also commendable to reward them for it. Imam Ahmad reported from Khalid bin 'Adi Al-Juhani that the Prophet, peace and blessings be upon him, said: "Whoever receives a favor from his brother without asking for it or eagerly anticipating it, let him accept it and not reject it, for it is sustenance that Allah has driven to him." Likewise, Ibn Hibban reported from Abdullah that the Prophet, peace and blessings be upon him, said: "Respond to the caller, do not reject a gift, and do not beat Muslims." If you wish to donate these funds on behalf of your employers, there is no harm in it. And if you wish to return them, apologize to them to appease their hearts, as Al-Nawawi said.