Click Modify.
Select the check box 
for each
thread you want to move to the archive.
Click Submit.
Using course tools>>Discussion Board>>Using archives
You can archive Discussion Board threads for future reference. To do so:
Enter the Discussion Board forum you want to archive.
Click Archives.
Click Add Archive.
Enter a title and description for the archive.
Click one of the options for displaying the text:
Smart Text displays the text as it is entered but converts URLs and e-mail addresses into links.
Plain Text displays the text exactly as it is entered, with no links.
HTML displays the text as you format it using HTML tags.
Indicate whether you want to make the archive available to students.
Note: Students can archive messages only if you assign forum administrator privileges to them. However, all students can view the archive if you make it available.
Click Submit.
Click Modify.
Select the check box for each
thread you want to move to the archive.
Click Submit.