Is it meant in the sense of "render a plan or intention incompletely"? (As opposed to "reduce from a present state of completion to an inferior state".)
I would think "miscarry", "founder", "confound", "forestall", "defeat", "waylay", "delay", "check", "sour", or "frustrate" could all be possible candidates.
For example, "a failure to precisely reverse will frustrate my return".
@Canaryyellow, an example of objects, consisting conventionally of parts, being reduced to a partial or inferior state, could be "undo", "break", "disassemble", "separate", "dismantle", "undress", even "strip" (of particular pieces).
Objects not consisting of parts, or reduced in a fashion other than disassembly, could be "degraded" or "damaged".
I really don't think there is one perfect antonym for "complete" in all cases, because of the variety of ways the word complete is used, and obviously "to uncomplete" is not in accepted use.
With most things there are a variety of ways in which they can be put in a degraded state so very generic words for reducing to an inferior or unwhole state (like "damage") tend to have negative and undesirable connotations rather than being merely neutral.
Your half-eaten apple isn't being "incompleted", it is being consumed or digested in the proper way, and so clearly any words with negative connotations are not suitable.
And what happens to the apple is very different to erasing answers from a test paper. The latter could be "reset".