Edit at the image’s original resolution first. If your camera does not natively produce 9000×6000 pixels, resizing up at the start will not create real detail and will mostly slow down your workflow.

For normal photographic post-processing—color, contrast, sharpness, retouching, dodge and burn, etc.—work from the original RAW file and make the best-quality native-resolution image you can.

If the client truly needs a 9000×6000 file, upsample as one of the final steps using a good resizing method such as Photoshop’s detail-preserving resampling. Also confirm what they actually need the file for, because clients often specify pixel dimensions or 300 dpi without understanding the intended output. The important requirement is usually the final print size or usage, not just the dpi number by itself.

In short: don’t enlarge first just to edit. Edit first, then resize for delivery if required, and set expectations with the client in advance.