The Replace Pages option can be found inside the Page Tab:
The Replace Pages window will open. The first thing you need to do is decide the source of the pages you would like to insert. You have three options to pick from.
From Blank Page.
This will allow you to add blank empty pages to your PDF. Select the number of pages you would like to add, then choose the page size.
This will let you insert an entire file into your PDF. If the file is not a PDF, Soda PDF will convert it as the file is inserted into your PDF. Click on the … to Browse your computer and choose your file. Decide if you want to add the entire document or if you want to take a page range from this document.
This will duplicate pages in the current file you are working with and insert the copies into your PDF. You can choose to insert a duplicate of all pages in the document or a specific page range.
Once you have chosen the source of the pages you want to insert, you need to decide which pages they will replace. You can choose the Current, First, Last or you can use Select Page to choose a specific page. When you are ready click on Replace.