If you wish to use the package manager for this (which is entirely reasonable -- it will take care of byte-compilation, ensure all the autoloads are processed, etc), then:
Copy the package (somewhere that isn't in your Emacs
load-path) and make whatever changes you wish to make.
If it is a simple package built from a single
.el file (ignoring
If it is a complex package built from multiple files:
Update the version string in the
If you're running Emacs 25+, invoke
package-install-file on the directory.
If you're running Emacs 24, create a tar file of the directory (include the directory itself; not just its contents), and invoke
package-install-file on the tar file.
You should be able to safely delete the unnecessary files from your modified source for the package (e.g.
*-autoloads.el, and for the simple package case,
(elisp) Packaging for more information.