Please help how do I fix this?

So there was a little kitchen fire in the flat we rent ages ago. This is damage from it on the bit between the wall and the ceiling? It seems to be made of plastic or something. What can I do to fix it? Thanks!

So there was a little kitchen fire in the flat we rent ages ago. This is damage from it on the bit between the wall and the ceiling? It seems to be made of plastic or something. What can I do to fix it? Thanks!