How to redirect your old url to your new one on tumblr?

The new tumblr doesn’t have the option ” use custom html ”
I have tried the codes, some of them and they do not work
I delete the html in the standard theme given and replace it with the code, save it, close it but then when i go back and customize the old html code is still there…
I have tried to domain name and replacing the page source.

Can someone please help
its frustrating me so much ! haha
THANKSSS
Answer