electricz0ltan is only partly correct. When you run FTL Dat Manager:
; navigate to your FTL folder; inside there is a folder called resources
and inside it a file named data.dat
; open this file.
The Dat Manager will now display all files and folders that are packed into data.dat
and export this file to a folder of your choice.
You can now open blueprints.xml with notepad and you can make adjustments. Be careful and keep a backup of the exported original because errors in this file will crash FTL.
Once you have made adjustments go back to Dat Manager, delete blueprints.xml and click Save.
Next click Add and navigate to your modified blueprints.xml file and add it to the pack. Save again.
(When replacing files it is important to first delete the file being replaced, and to add the modified one in the exact same folder the original one was. For data.dat this is not so important but for resources.dat it is.)
Now when you run FTL it will load the modified data.dat with the modified blueprints.xml file in it.
It is in your best interest to make backups of data.dat otherwise you may have to reinstall the game in things go badly.