Go to File menu in Lightroom > Open Catalog > Choose the 2016 catalog that you want to upgrade to Lightroom 6.
I tried that but it's making me select each picture one by one. It won't let me import the entire catalog.
Side note: I think I have gotten a workaround figured out.
Do you mean to say that it allows you to upgrade each catalog one by one?
And you want to merge all those catalogs into one?
No. I have to select the catalog, the select the folder, then select a file and import. Repeat 3000 times.
I did find a way to get around it, so my problem is solved. Thanks for the help though Chetna.