I presume that you mean Contributor rather then Author.

1. Click Site Settings in the top nav

2. In SharePoint Portal Server: Click Manage security and additional
settings
In Windows SharePoint Services: Click Go to Site Administration

3. Click Manage site groups

4. Click on the Site Group

5. In the toolbar, click Edit Site Group Permissions

6. Uncheck the right that you don't want that site group's members to have
and click OK

I'm not following your instruction. It may be that we are
using different version of Sharepoint. We are currently running
Sharepoint Portal Server 2001. There are three privilege roles,
reader, author  & administrator. Currently a reader is able to
add, and delete  files and folders. I really need to change this so
that they can add/post but not delete.
I'm not able to find anywhere to manage the security of a role.
Your thoughts???

I don't have SharePoint Portal Server v1 available to give instructions.
Perhaps someone else can help you.

I don't belive there is a way to do that without customizing the underlying SharePoint files in SPS 2001. One of the big benfits of SPS 2003 is the more granular control of role permissions and the ability to create custom roles. SPS 2001 was very limited in this respect.

U can customize the group permission by (site settings->go to site administration->manage site groups) this can be done easily in SPP 2.0, and as u said u r using 2001 version, I am not sure about that.