FAR 52.212-4(c) addresses the actual revision to one or more contract terms or conditions; in this case, the new completion date. FAR 52.212-4(f) addresses the issue of liability for inability to meet contract requirements. The two paragraphs address separate concepts, which in this case have their roots in the same basic issue (Covid). So the actual authority to change the period of performance is in para. (c). The contractor's protection from liability for not being able to meet the completion date is in para. (f).