How Do I Change iTunes To Another Hard Drive?

Hi everyone. I have an iMac with two internal hard drives. A 250GB and a 1TB. The 250GB is already full. Only have 5GB left. The 1TB has 422GB free of space. In some applications such as iTunes, I’m not able to do a simple back up of my iphone because it tells me that I don’t have enough space in the computer. I guess this is because iTunes is installed in the 250GB that has no space. How do I control where iTunes saves backups?
Thanks
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Lorenzo