This week old post by AMK Mapping explains how Russia achieved success in Kupyansk:

In the Kupyansk direction, over the past two weeks, Russian forces have made significant advances in Kupyansk City, and are now threatening to encircle the remaining Ukrainian formations in the northern suburbs.

In the west, Russian forces consolidated control over their positions in the southwest of the city, and expanded their zone of control into the neighbouring residential streets. They also began attacking to the southeast, bypassing the Yuvileynyi District from the east, approaching the southern outskirts of the city. Additionally, they were able to capture further residential areas in the area of the highway, towards the railway line, threatening to cut off the Ukrainian groupings further north.

In the east, Russian forces broke through Ukrainian positions in the city centre, and quickly advanced south to the bridge over the Oskil River. They were able to capture the Kupyansk Administrative Building, as well as most of the rest of the buildings that make up the city centre. The Russians were also able to infiltrate into the market, just south of the highway, but it's unclear if any consolidation has taken place.

+ ~1.91 km2 in favour of Russia.