We have 2 sites of one restaurant. The old one is Premium and on the domain. I prepared new desing on new site for this restaurant. Now I need to put the new one into the old one - because of Premium and the domain. Is that possible or is that kind of process wrong? Thanks 
Someone else might have a better answer, but I’ll just leave this here.
If I understand correctly, you want to use the new designed website and stop using the old design. If that is correct, I think the easiest thing to do would be the following.
I would recommend moving the premium subscription to the new site and moving the domain over to the new site as well.
Please be aware that if you do choose to do this the information stored on your old sites dashboard may not appear on your new sites dashboard.
If you choose to use this method, I am not responsible or liable for anything that may go wrong.
I’ve attached links to some articles in the WIX Help Center to explain how to do this. I’d recommend you read through these and make sure this is what you want to do before doing anything.
I hope I have been of help.
Assigning a Premium Plan to a Different Site in Your Wix Account - https://support.wix.com/en/article/assigning-a-premium-plan-to-a-different-site-in-your-wix-account
Transferring Your Wix Domain to a Different Wix Account - https://support.wix.com/en/article/transferring-your-wix-domain-to-a-different-wix-account
Moving a Connected Domain (Purchased Elsewhere) from One Wix Account to Another - https://support.wix.com/en/article/moving-a-connected-domain-purchased-elsewhere-from-one-wix-account-to-another
Thank you Noah. 
Yeah, it’s working <3 Thank you so much!
No problem, glad I could help.