I can't replace the world of realms I signed. Help me, please.

Hi, I have a problem with Minecraft pocket version and I need help. I created realms for up to 2 friends and selected an existing world, but when I joined the realms the world was different (that didn't exist), without coordinates. I've been trying to replace the world of realms for hours, for the one I selected at the beginning but always says "there was an error" and won't let me change it. But I already signed the realms. What do I do?