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Go to system admin > modules > and change the order number for the forum module to 0 to keep it from showing up in your main menu. Then go to blocks admin and click edit by the main menu block and then click edit template and add in the link to your new forum. Just make sure you use the same class for your newly added link as the other links are using.
For example you would add your new link like this:
ForumThis will only work if you have cloned your default template. To do that, click on templates in your system admin and click on the clone link by your default template and name it default new or whatever you want. Then go to preferences > general settings and change your default template option to the new default template you created. That will allow you to directly edit the templates for your blocks.
