Hello Hazel,
I am sorry but unfortunately the view all link isn't configurable currently but you can of course create a template override for the magazine layout and modifying the link.
For instance, if you are using "layout_1", edit the file /components/com_easyblog/themes/wireframe/blogs/magazine/layout_1.php and replace the following codes,
[gist]
<a href="<?php echo $viewAll;?>" class="eb-more__btn"><?php echo JTexT::_('COM_EASYBLOG_VIEW_ALL_POSTS');?> <i class="fa fa-chevron-right"></i></a>
[/gist]
With,
[gist]
<a href="<?php echo EBR::_('index.php?option=com_easyblog&view=categories&layout=listings&id=9999');?>" class="eb-more__btn"><?php echo JTexT::_('COM_EASYBLOG_VIEW_ALL_POSTS');?> <i class="fa fa-chevron-right"></i></a>
[/gist]
Remember to replace the `9999` with the correct category id.