(How) can I changed the handler to enable editing of previewed PDFs?

My default preview handler (Microsoft PDF) enables viewing but not editing.

Most PDF viewers are read only. PDF-XChange Editor may be an exception, but I haven’t tried it myself.

The last one installed is usually the one which will be used, although you can override it. Finding a PDF preview handler which supports editing will be the main step.

How do I override?

Worry about that after confirming there is one that can actually do editing.

The one you installed last (or chose via Open With, in some cases) will usually be used by default. (They tend to take over the registry settings from the other PDF viewers each time you update or install them.)