Learn English – Which is correct: “full context” or “complete context”

It depends on what you are trying to do, and the broader context:

• If you are simply stopping the process before completion Cancel would be appropriate
• If you have made a change but want to back out Undo would be appropriate
• If "Complete" is a record state, you have set a record to complete, and want to change that then "Make Incomplete", or "Back Out" would be appropriate.

In the last case it might also be possible to simply label the button "Incomplete", or have some sort of state combo to change the state, and then have a save button to commit.

However, there is no direct antonym to "complete".

FWIW, in user interface design it is often more appropriate to use a common idiom than it is to be strictly precise in the wording of your button.

They're both grammatically correct and accepted.

For me though, and if you would like to dissect the nuance between them,

Both of them mean a "person to whom thought or action is directed"

But object emphasizes:

"a person seen as a focus for feelings, thought etc."

ex. an object of affection/ contempt

While subject:

"one who experiences or is subjected to something"

ex. the helpless subject of their cruelty/ ridicule