To relink a file in Adobe indesign.
Window → Links
In links it will display all of the external links that are in your document.
Links is all the information from the outside of indesign for example; pictures, imported body text, video and hyperlinks. All of this data isn’t saved directly onto a Indesign document to reduce the files size. You can also change the links from this window.
Here is an example of a picture I have in an Indesign document. It clearly displays the file name, mine being Gabrielle Aplin .jpg, to change this picture quickly you would need to select your image in the document and then click the link button 
. This will open your documents and you select the desired image to then effectively change it.
Saving and Packaging.
The standard DPI that is industry approved is 300 with that knowledge you can compress files that clients send you to these standards as anything higher is useless.
Getting your file ready for print.
Packaging a file collects all your picture and fonts together into one place so they can be exported out to other places such as printing companies. To do this you go to:
File → Package
To export PDF files you go to:
File → Export → Adobe PDF (Print) → Adobe PDF Preset and Select → High Quality print.
Both of these methods compress your images into one easy folder.
