Depends on how they are marked as invisible.
The two primary ways are to have the hidden bit set, or by prepending the file name with a .
For the first, use the Terminal command, chflags
The syntax is,
chflags nohidden <file>
If you need to rename a file, use the mv (move) command.
mv .hiddenFileName unhiddenFileName
Now, what is it you want to do that you need to unhide files? There's likely an easier way.