Landcore is different from the other types of Landplot.
Landcore cannot be removed from your island until the island is completely delete
Landcore can only be replaced by the other one
Step 1: Click on "Edit Land" button in bottom left corner of screen.
Step 2: Click on Landcore in the island.
Step 5:
Click on "Apply" button to save your change
or
Click on "Reset" button to reverse your change.
Now you know how to modify your blueprint.
Step 3: Click on Blueprint from Inventory that you want to modify.
Step 4: Click on Modify" button.
Step 5: Click on Landcore in the island.
Step 7: Click on Landcore in inventory to replace with the old one
Step 8:
Click on "Apply" button to save your change
or
Click on "Reset" button to reverse your change.
Now you know how to modify your blueprint.