Section name change after migration

I have just imported a blog from another platform and unfortunately it has renamed the urls from to

I want to keep the existing backlinks pointing to /post/article-name/ but have the content redirect to /posts/article-name/

I thought this could be achieved by using something like

    posts = '/post/:slug/'
    posts = '/post/'

but seems I have misunderstood how this works? Can anyone point me in the right direction please?

What is “it” ?