problem with URL

Hi there, I just downloaded WordPress (using MAMP software) and the URL that was created for my site URL and wordpress URL was https://localhost:8888/mysite. I went onto settings-general and tried to change the URL to one that I wanted and clicked save and it said that the webpage couldn’t be found. I tried to go back to my dashboard but that URL has changed. I went to PHP my admin and under site URL and home URL, it has been changed to the new URL that I wanted and looks like it has worked (?) but then when I type that URL into the address bar, it says it can’t be found. Right now I can’t even access my dashboard as neither URL work – what should I do?