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? If only you had something they want. 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!

Details
For manual upgrades, instead of upgrading right away, the plot should be marked as upgradeable instead.
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 50% and 150%).

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.

Upgrade costs:
Agricultural: lvl2: 10000, lvl3: 20000, lvl4*: 30000
Commercial: lvl2: 20000, lvl3: 40000
Industrial: lvl2: 30000, lvl3: 60000
Martial: lvl2: 12000, lvl3: 24000
Recreational: lvl2: 15000, lvl3: 30000
Residential: lvl2: 5000, lvl3: 10000
Advanced Industrial: lvl2: 40000, lvl3: 80000

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* Does NOT bypass the requirement for this