The Replace 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 replace pages of your document with blank empty pages. Select the pages you want to replace, the number of blank pages you would like to add, then choose the page size.
Within Current 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 All or Range. When you are ready click on Replace.
From External File
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 Browse source file 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.
When you are ready click on Replace.