As @lawlist said in a comment, use the
dired-do-chmod command (which is bound to
M). This asks you what mode you want to set on the file, and it expects either a three or four-digit octal number or a symbolic representation of the mode. This is exactly the same value you would give to the chmod program if you were running it at the command line. The difference is that
dired-do-chmod works in windows, even though permissions in windows work a different way.
Using an octal number to represent permissions is concise and traditional, but hard to remember and explain. It's basically a bitmask of the permissions, and
660 are the usual values that you would use. Changing that to
440 will unset the bits that allow writing, thus making the file readonly. I won't explain further here, but you can look it up if you're curious.
A better way to do it is to to use the symbolic representation. If you use
a-w, then you are saying that for all users you want to remove the write permission. Again I won't explain all the possible values you can use here.
This is slightly overkill for just making the file readonly, so you might think about recording a keyboard macro and saving it for future use.