There are a few different reasons you might want to redirect a page URL in Squarespace:

If you’re redesigning your website on Squarespace, you may need to update page URLs from your old site like /about.html to /about. If you don’t want to lose your search ranking, you can use a 301 redirect to pass the old page’s ranking to the new one. This is super easy to do in Squarespace and I recommend it any time you are permanently changing a page URL.
If you delete a page, you can set up a redirect to any other page on your site to prevent visitors from seeing the 404 Error page.
You can also use 301 redirects to pretty up your links. So, let’s say you are using a URL for your contact page that includes your keywords, like /austin-wedding-catering but you want it to be easier to give people the link. You can set up a redirect so that your client can type in yoursite.com/contact and it automatically redirects to /austin-wedding-catering.

In Squarespace, this is done through URL Mapping.

Step 1.

Make a list of the pages you will be redirecting, their old URL, and their new URL. I have a handy spreadsheet I’ve created to help with this. It automatically gives you the code you’ll need for URL mapping when you enter in the old and new URL slugs. Get a free copy here.

Step 2.

Copy the cells under the URL MAPPING results (you can drag your mouse to select all of them). Next, on your Squarespace website, navigate to Settings > Advanced > URL Mappings. Paste in the URL mappings. They should look like something this, with each mapping in its own row:
/about.html -> /about 301
/contact -> /austin-wedding-catering 301

PRO TIP

If you are redirecting a collection like a blog, you can sometimes do this in one fell swoop. If you are changing just the base URL of the blog, you can redirect all the posts using the [name] parameter like this: /oldblogURL/[name] -> /newblogURL/[name] 301

Step 3.

Save your work, then test it out. Go to yourdomain.com/oldURL and make sure it automatically forwards you to the new page. If you have any trouble, check out the official Squarespace URL Redirects support document.

Step 4.

After you’ve finished setting up and testing your redirects, be sure to index (or re-index) your website in Google Search Console so that Google knows to crawl your site for the new page information. Then Google will update its search results to show your new URLs.

DON’T PANIC

When Google indexes your site, you may see a warning or error like “Indexed, though blocked by robots.txt.” That is normal behavior because Squarespace tells Google not to crawl certain pages on your site that are just for internal use.