Manually.
If you’re running the free version, you’re going to get post as results. So, you have to edit those posts. You can use relevanssi_content_to_index to add whatever data you wish to posts before they are indexed by Relevanssi (two arguments: the content to add and the $post object). Just add a function that figures out which taxonomy terms are assigned to a post, then find the meta data for those terms and add it to the post content.
With Relevanssi Premium, you can get taxonomy terms as results. In order to add the meta data to the taxonomy terms, the filter hook relevanssi_tax_term_additional_content is what you need (again two arguments: the term description and the whole term object).
Here’s some additional information about this: https://www.relevanssi.com/knowledge-base/indexing-additional-content/
