How to remove remote management in iphone without password?

It shows that “ these phone is supervised and managed by” one day earlier my phone got automatically switched to screen recorder and I not did that, this is the used mobile of someone I don’t know, he sells this mobile to a shop and then I buyed this iphone 6 128gb.it also shows that they can access my mobile or they can install apps as well delete the apps and deleting data etc. Please make my mobile safe and make make my information safe.