WordPress Free ThemeCopyright

Hi!
I am confused about liscense of wordpress. On Quora and some other places I found that wordpress if free and open-source (which is obvious) and we can edit/remove footer default thing ( proudly powered by wordpress).
I have installed a FREE wordpress theme and now can I remove/edit the footer material or remove the default credits which are ( powered by wordpress | theme by <company>). Is it legal to remove.edit this footer of free theme?
Shall be very thankful to you.