How can I replace the sealing flange from the toilet bowl to the sewage pipe coming from the side wall?

If I understand your question correctly, you have wall-mounted toilet and want to replace wax ring, correct..

Depends on what kind of toilet it is but most of wall-mounted toilets use "donut" seals. Some brands, like Geberit, use rubber washers with gills. To replace the seal, you will have to dismount the toilet, remove old ring, insert new seal and reset the bowl.

Back to you. Milo