When my living room window blinds were opened, the piano sound was much improved and more rounded.
Similarly, I have noticed a better sound since it was put up against the wall.

How can one further improve the acoustics of the room (alter external things) to improve the quality of the upright piano sound?

One thing that worked for mr corenfa was to stuff a duvet down the back of the piano (between piano and wall). This of course only works if you have the same problem that he did: piano was in a room with wooden floor, and sounded too bright. I expect there will be other solutions depending on what your circumstances are.

I am no expert but have just observed a few things over time.
Soft furnishings, books, duvets and people tend to absorb sound so the more of these things you have around the more subdued the sound will be.
Tiles, plastered walls etc tend to just reflect the sound without any sympathetic vibrations so will deliver a brighter,colder sound.
I like wood as an acoustic surface but not everyone in my house/street agrees (we recently put down a wood floor and the piano sound now really fills the room). Curved wood as in certain opera houses and concert halls is best of all, but not easy to get into the design of an average living space!

So if you prefer a softer sound introduce rugs etc - even a small carpet under the instrument pedals makes a difference I 've found. Otherwise remove them. Experimentation will be key.

Due to lack of space, I had to raise five chicks in my piano teaching room until they were a month old. The sound and smell is now a whole lot better!

Open the top of the piano?

Courtesy of the previous owners of our house, who clearly felt that they were living in a Mediterranean country, we have ceramic tiles on the floor of our lounge/dining room which also houses the piano. Consequently the sound is pretty bright, although the addition of some carpets and a 3 piece suite has balanced things out a bit.

We have tiled floors at school and I stand the piano on a piece of carpet but can't do much else. In the tiny room where I teach it is reasonable though the glass window pane behind it does vibrate in damp weather. I don't know why.

For exams I have to use another, bigger classroom, but the floor, again, is tiled. This makes for a rather bright sound. I do sometimes wonder if examiners take into account the acoustics of the room in which exams take place and the effect this will have on various instruments - and the voice.

I did once investigate how I might improve the acoustics of the cantine where we do our concert. The ceiling is low and has ceiling tiles which deaden the sound, the floor is lino tiles but the space is big and there is one glass wall. I found that you can buy wooden acoustic panels of various shapes and sizes to put behind or round your piano but I don't think they would do for a living room and they were very expensive. I made do with some plywood screens which belonged to the school canteen. They did help to keep the sun out but didn't make much difference to the sound.