Recasts to focus on Aspex and Delivery Hero; adds price details, context and Aspex founder comment in paragraphs 2-5.

May 11 (Reuters) - Activist investor Aspex Management has raised its stake in German food delivery group Delivery Hero DHER.DE to about 15% after buying a 5% holding from Prosus PRX.AS for around 335 million euros ($393 million), increasing pressure on CEO Niklas Oestberg.

Prosus' parent Naspers NPNJn.J said on Monday the Hong Kong-based asset manager had paid 22 euros per share, a 10% premium to Delivery Hero's closing price on Friday and about 22% above the stock's 30-day volume-weighted average as of May 8.

The sale reduces Prosus' stake in Delivery Hero to 16.8%. It is required by EU antitrust regulators to cut its holding to below 10% by late summer, as part of conditions tied to its takeover of Just Eat Takeaway.com. It previously sold a 4.5% Delivery Hero stake to Uber UBER.N for 270 million euros.

Aspex has urged Delivery Hero, in a letter seen by Reuters, to withdraw from entire regions and replace Oestberg. The increased stake gives it more weight before Delivery Hero's June 23 annual general meeting, where Prosus cannot vote on its shares, as they are being managed by a trustee.

"As long-term shareholders, we have increased our stake, reaffirming our belief in the value upside for the company. We will continue to work constructively with Delivery Hero's supervisory board and management to enhance value for all shareholders," Aspex founder Hermes Li said.

($1 = 0.8512 euros)