To change a plants Yield settings go to the plants blueprint so for example the BP_TomatoPlant and select the BP_PlantComponent in the Components panel, then in the Details panel and inside the Plant Settings Category find the Yield category like the image below.
Here you can change this plants Yield settings, you can hover your mouse over each settings name to get a tooltip with more info about each setting.
If Use Yield is true the plant will use its yield value to determine how many vegetable resources the player will receive when the plant is harvested. The higher the current yield the more vegetables the player will receive when they harvest the plant. Max Yield is the max value Yield can be. This amount will still be confined to the min and max amount values you set in the vegetable resources setting.
Starting Yield is the starting Yield value. Note – Yield can go down as well as up if yield is 0 then the min amount resources will be collected when the plant is harvested.
If Yield Starts when Full Grown is true then the Yield value wont change until the plant is fully grown.
If Yield Stops when Vegetable Fully Grown is true the Yield value will stop changing once the vegetables are fully grown.
Stats can effect the plants Yield and increase or decrease it, you can find these settings in each stats category in the plant components settings like the example of the water Yield settings shown in the image below.
If Does Water Effect Vegetable Yield is true then the value set in the increase and decrease Yield settings will effect the plants Yield.
Vegetable Yield Increase is the amount the Yield value will increase if the plant has > the Min Water Yield Increase amount of water.
No Water Vegetable Yield Decrease is the amount the Yield value will decrease if the plant has no water.
If Show Yield UI is true the current Yield value will display in the plant UI.