Extract the wsp file through any extraction application like winrar.
Make desired changes and save.
Download MakeDdf file and place it in the folder where wsp was extracted, open command prompt and type the following command and enter.
C:\extracted wsp>makeddf /p wspname /d wspname.ddf /c wspname.cab
Then, run the following command.
c:\extracted wsp>makecab /f meow.ddf
Then rename the file to the following so as to utilize it again.
c:\extracted wsp>RENAME wspname.cab wsp.wsp
Your WSP would be ready to be deployed.