Extract you use dpkg with "-x'
Example
dpkg -x somedebfile.deb /home/user/extracted/

That will place the content in /home/user/extracted

You can add, remove files in that directory, then use dpkg-deb to rebuild a deb file.
You will need to create a DEBIAN folder in /home/user/extracted which contains control file and changelog file.

I suggest looking up instructions for creating deb files for more information on this.