The standard answer for this type of question is, google it!
A simple google search returned this article as the first hit:
Set file attributes: Hidden, ReadOnly[
^]
Granted, you would have to know how to change the word true to false, but you should be able to figure that out.
And next time, try to make your title fit your question. The title looked like you wanted to "lock" a file, not change it from read-only.