If you're looking to change your URL on Tumblr, there are a few different ways to go about it. The first and most common method is to simply log into your account and click on the Settings icon in the top right-hand corner. From there, scroll down to the Blog Address section and click on the Edit button next to your current URL. You'll then be able to enter a new URL of your choice.
Another way to change your URL is by using the Customize Theme option from Tumblr's settings menu. Once you're in the customization interface, look for the Blog Title & Description section and click on the Edit blog address link. This will bring up a window where you can enter a new URL for your blog.
One last thing to keep in mind is that you can only change your Tumblr URL once every 45 days, so choose wisely!