i wanna modify some files in .deb package and upload it to my PPA, is that possible and how to ? Thanks in advance
No, this is not possible.
A binary Debian package contains built binaries only and there is no way to retrieve sources from binaries as a lot of information is lost during the process.
If you were to edit only data that are not modified by the compilation process (say configuration files for instance) then you would be able to generate a new .deb file from the old one. However, you will not be allowed to upload this one to your PPA for safety reasons. All the packages in the PPA's must be compiled on the servers sandboxes to prevent malicious modifications of packages. It does not prevent people from possibly building dangerous Debian package, but at least it guarantees that the provided binaries are really the output of the source code compilation process.