Probably the easiest way is to add a new text element manually for the marriage details (it can contain any dummy text) and link it to the correct data in the Legacy database using "Format | Database link" http://treedrawlegacy.spansoft.org/manual/html/fmt_link.php
Then use "Edit | Update text" to import the actual marriage details.
The alternative is to open your existing chart then import a small tree consisting of just the couple you want to update. This can by done by setting the number of generations to import to zero http://treedrawlegacy.spansoft.org/manual/html/import_options.php
Then you would need to replace the existing info block for this couple with the new one. There's a video showing how to combine two trees at https://www.youtube.com/watch?v=_IZDxnHtJ6A