The cat command can be used to retrieve any revision of a file:
$ hg cat -r 10 myfile.pls
You can redirect the output to another file with
$ hg cat -r 10 myfile.pls > old.pls
or by using the --output flag. If you need to do this for several files, then take a look at the archive command, which can do this for an entire project, e.g.,
$ hg archive -r 10 ../revision-10
This creates the folder revision-10 which contains a snapshot of your repository as it looked in revision 10.
However, most of the time you should just use the update command to checkout an earlier revision. Update is the command you use to bring the working copy up to date after pulling in new changes, but the command can also be used to make your working copy outdated if needed. So
$ hg update -r 10 # go back
(look at your files, test, etc...)
$ hg update # go back to the tip