First, go to Workspace settings, which you'll find under your workspace name in the top left corner:

Then, scroll down and click Rename workspace:

Note:
At the top of the same workspace settings page, you'll see options to change your workspace logo and workspace URL.👇

Simply upload any image here to use as your workspace logo. This adds a nice branded touch, and makes it easier to choose between workspaces as you navigate:

All links to pages in Whimsical contain your workspace key so you can easily identify which workspace a link belongs to. Like this:
You can change this workspace key at any time. Workspace keys must:
Important: If you have SAML enabled in your Whimsical workspace, the workspace key can be used as a landing page, like this: https://whimsical.com/@:workspace-key
So if you change it in Whimsical, you'll need to update it in any other systems or bookmarks where you have used it.