Here's the steps to change the url of your youtube channel.
Sign into your YouTube account.
Go to advanced account settings by clicking your profile in the top left corner, then click the settings icon (It looks like a gear). Then click Advanced.
Under “Channel settings,” select the link next to You’re eligible for a custom URL. This link will only appear if your channel is eligible for the custom URL.
In the “Get a custom URL” box, you’ll see the custom URL you’ve been approved for. You may need to add a few letters or numbers to make the URL unique.
Check “I agree to the Terms of Service,” then click
This will only work if your youtube account is eligible.
Here's the requirements to be able to change your youtube url.
500 or more subscribers
Channels is at least 30 days old
Channel has uploaded a photo for the channel icon
Channel has uploaded channel art