It says "everywhere" that one should shut down virtual machine to change memory allocation. But there is no explanation whatsoever how to do that. So I had to search myself. After a long time I eventually found this way to do it:
- Click parallels icon
- actions
- shut down
- Wait while windows turns off
- Wait for black Windows window to turn up on the screen
- click tools icon
- hardware
- CPU & Memory
- Change the amount of memory
- Voilá!