Go to your Facebok Business page and go to the Manage Page menu on the left.

Look for Meta Business Suite.

how to find facebook meta business suite

Next find the Content tab on the left hand side menu.

meta business suite content tab

Select a post that you want to share and click the 3 dots and click on 'Duplicate post'.

meta business suite duplicate post

Now you can share your old Facebook post to your Instagram. Super easy.

share facebook post to instagram