Change the current domain name to another?

Hello guys, I am totally new here and i don't know where is where... The past few days I am trying to figure out how to make some changes in my friends website that asked for help, but it seems i need help too. So, the issue here is that i have an old Domain name & it's redirected to a new one. I don't want to use the new Domain name that is connected with redirect to the "old" original Domain name, but i want to keep the new data from the new Domain name and insert them somehow to the old original Domain name. As a result, I want to delete the old data that are at the moment at the old Domain name(so the old site is obvious without redirect) and take the new data from the new Domain name(that is now obvious due to redirect) and insert them to the "old" original Domain name and keep just that. How should i make it happen? Could you please give me some instructions? Another small issue here, as the website wasn't originally mine and as it seems it was poorly made, i can't even realize which file is from the old site and which from the new..Any way to realize that as well? I opened the code and did some trials, but it seems that somehow some files are connected through the old site and some through the new. So, it's not really clear. If it was just one file with everything it would be a lot easier. I already asked my friend,but has no clue from IT and the person that made it is not available at all. To conclude i would like to achieve the below goal: Keep old domain name & New data to "old" Domain name & cancelling old data & cancelling new Domain name that is now connected with redirect. *//Keep domain.com, cancel domain.com/domain cancel previous domain.com data insert domain.com/domain data to domain.com//* Thank you all in advance for your time!!! :)