China is easy. Even after the First Sino-Japanese War, the Twenty-One Demands, the invasion of Manchuria, etc. many Chinese, especially elites, viewed Japan positively. Some of it was Pan-Asian sentiment, though more of it was admiration. Many Chinese studied in Japan and saw it as a model for Asian modernity and resistance against Western imperialism. It was only the traumatic national experience of the Second Sino-Japanese War that completely destroyed Japan's reputation in China. There are many PoDs that can prevent the Second Sino-Japanese War or make it less brutal than it was IOTL.
Korea is harder. You need to avoid the colonial experience entirely. And before someone says "but look at Taiwan!", keep in mind that 1. the Taiwanese colonial experience was very different from the Korean one and 2. Japanese rule was not viewed nostalgically until Taiwan experienced decades of brutal dictatorship under the KMT, and in fact the KMT was initially welcomed enthusiastically by the people of Taiwan. The circumstances can't be replicated in Korea. No, I think for Korea to view Japan positively you need to prevent colonization.
The best way to prevent both the Second Sino-Japanese War and the colonization of Korea is to nip Japanese imperialism in the bud. I'd look at something like a Russian victory in the Russo-Japanese War (easily achievable). Without a history of violent expansionism, Japan will be viewed much more positively by Koreans and Chinese. There will certainly be rivalry, but it will be of a friendlier sort.
If you're just looking to improve postwar relations, the best way is change leadership in South Korea and China. Both Park Chung-hee and Mao were eager to re-establish diplomatic and economic relations with Japan as a way to boost their own impoverished nations, something that was not popular with the people at home. Reparations were either not paid in any significant matter (in China's case) or pocketed by corrupt government officials (in South Korea's case). This has both inflamed public opinion in Korea and China as well as given Japan an excuse to not seriously address the crimes they committed since in their view they already have sufficiently apologized and made up for it. A situation where the governments are more willing to press for reparations and apologies in the postwar period would probably do a lot to settle grievances. Maybe less brutal Chinese Civil War and Korean War? In fact preventing the Korean War (if such a thing is even possible, which is a debatable topic) will seriously slow down Japan's postwar economic growth, and would disincentive the actions that the South Korean and Chinese governments took IOTL. Alternatively, you could create a stronger Japanese postwar social movement pushing to address the nation's criminal past in the way that you had in Germany with the 68 Generation, leading to earlier and more genuine gestures of apology and some sort of Japanese Ostpolitik. I don't know enough about the details of Japanese postwar social movements to give a concrete PoD for this though.
