How can I rebuild an edited perl6 module that I've downloaded?

As it has been noted already you should not modify installed files. However, the workflow for testing changes of some module is pretty straight forward.

First:

# Fetch and extract the distribution, then change into its directory.
# If the distribution is still in zef's local file cache then it will
# skip the fetch and extraction steps.

zef look Time::Duration

Then:

At this point you can edit any of the files you wish.

Finally:

# test your changes without reinstalling (assumes all dependencies are installed)
zef test .

# Reinstall the modified version of Time::Duration using one of the following:
zef install . --force-install
zef uninstall Time::Duration && zef install .

You should git clone the code or download the zip package, edit the code you want, then zef install . if the current directory have a META6.json file.