Correct spelling, explanation: this form, replacing, is the correct one because the root word replace ends with e. In such a case, when creating a gerund form, this last e is omitted, so replace becomes replacing. For the same reason, replaceing is a mistaken form.

Definition of replacing:
verb, gerund form of replace, to provide a substitute in the place of sth/sb
They are replacing some people in our company with different positions, due to changes in the highest branch.
What do you think about replacing white walls with blue ones in our bedroom?

Incorrect spelling, explanation: replaceing is an incorrect form because e from the end of the root replace should be skipped when adding -ing suffix, as in replacing. This is always happening in the case of verbs ending with e, therefore the correct form is replacing.

Incorrect spelling

Incorrect spelling

Incorrect spelling