All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is forbidden to violate someone else's Wi-Fi by using it without their permission. The religious texts emphasizing the prohibition of violating other's properties and rights are numerous. For example, Imam Ahmad recorded in his Musnad from Abu Hameed As-Saa'idi that the Messenger of Allah said: “It is not lawful for a person to take the wealth (property) of his brother without his right (i.e., unjustly).” This is because Allah has forbidden the property of a Muslim to be taken by another Muslim unjustly.
And what is incumbent upon you is repentance to Allah for what you have done. One of the conditions of repentance is to rectify the situation with the rightful owner. Therefore, you should compensate the owner of the WiFi for the value of your use of the network without his permission, or seek forgiveness from him. You are not required to delete what you have saved of videos and other applications.
Allah knows best.