If I understand what you are trying to do correctly, the problem is your last event is telling your game to delete all the cornplant2 and strawberry instances whenever you click on the corn image. You need specify which instance of these you want to delete when you want to replace them with a different crop.
top level event:
eg. on left button clicked on corn_image2
layer "cropselection" is visible
pick placement location instance with UID = whatever
Sub events:
if placementlocation is overlapping cornplant2 - destroy cornplant2
if placementlocation is overlapping strawberry - destroy strawberry
blank - spawn cornplant2
- set selectedlocationID to -1