Bulgaria Air Customer Service Issues

I have sent several emails, with the last one on 11/5/22, but have not received any responses. I've attempted to contact Customer Service around 20 times in the last 3 weeks without success. Could someone please assist me? My primary issue revolves around my booking with Bulgarian Air for a flight from Sofia to Varna on 27/5 and back on 3/6, both under PNR NFW56N. Due to a rescheduling of my connecting flight from Malta to Sofia with Ryanair on 27/5, I will now be traveling to Varna via Vienna on 26/5. I wish to cancel the flight on 27/5 but still intend to use the return flight on 3/6. My worry is that if I cancel or miss the flight on 27/5, the return flight on the same PNR might get automatically canceled. How can I cancel the 27/5 flight while keeping the 3/6 flight valid?