Hello.How to erase my old backup from icloud? i just switched to another iPhone and i cant erase my old backup from icloud.it says it couldnt perfome it at that momment.

Hello.How to erase my old backup from icloud? I just switched from iPhone-X to iPhone 11 Pro Max and i have 50GB of icloud. The problem i have now that my icloud wont allow me to erase my iPhone-X backup. It says it couldnt perform it at that momment. Now i'm running out of my 50GB icloud. My old backup takes around 13.2GB space. I wish not to upgrade my icloud at this momment since i really have enough space consider i could delete my old iphone backup. Can anybody please help?