There is a plugin way – and a coding way…
I will explain both
Plugin version is good for individual posts / coding way is a mass archive way…
PLUGIN VERSION
Using a plugin called WordPress SEO by Yoast.. you can go into the previous posts and do the following.
Open the POST dashboard in your WordPress blog.
Find the post/s you are interested in working with.
Click ‘Edit’ post
Then scroll down to the WordPress SEO by Yoast section on the bottom anc click ADVANCED
From there.. you can select noindex / nofollow for that post – as well as a few advanced meta tags.
You can do this for any or all of your posts… 
CODING VERSION
Step 1
In your theme folder locate the following files:
/wp-content/themes/yourtheme/
Archive.php
Header.php
Open them both up.
Step 2
First thing to do is SAVE the header file as:
header-archive.php
with that new version open add the following code into the header section:
Then save it.
Upload that file to your wp theme folder online.
Step 3 now go to the open archive.php file and find and replace the following:
FIND:
REPLACE WITH:
Save and upload that file to your theme folder as well.
Now you have just MASS added a NOINDEX / NOFOLLOW to all of your archive pages – but NOT the individual post pages themselves.
Hope this helps!
