Sometimes you just can't wait for a plot to upgrade. You need that upgrade RIGHT AWAY, HERE AND NOW, AT THIS VERY MOMENT!!1 But, how do you
convince these lazy settlers to stop slacking off and get right to building again? (Or to get the necessary scrap they might be missing, whatever's the case...)
If only you had something they want (or could use to purchase building materials with). If only you could put these literal mountains of bottlecaps you keep carrying around to some use...

Howto
Go to your City Manager's Desk. Craft an "Investor's Clipboard" from under "Tools". Then, just walk to the plot in question, and use the clipboard from the "AID" section of your Pip-Boy. If you have enough change, you can "Invest" in the plot, prompting it to upgrade right away!

This also works to start a plot's construction, if you don't have enough resources.

The amount of bottlecaps you need to invest is based on SS2's scrap requirements. The more resources a construction requires, the more expensive it will become.

You can also craft an "Investment Config" holotape from under "Holotapes"
in the City Manager's Desk. There, you can set a cost multiplier for the investment costs (between 0% and 200%).

If you have MCM installed, you can also adjust the cost multiplier from there, and set a hotkey instead of using the aforementioned AID item.

Limitations
One of the reason this took so long is that Sim Settlements 2 provides no easy way to insta-upgrade a plot. Even now, it uses a testing command for addon developers, and refreshes the plot afterwards. There will be no intermediate construction stages.