How can I go back to my ISP's DNS in my Airport Time Capsule after I've changed original DNS? Thank you.

And the next simplest way is to search for your ISP’s posted settings, or to contact your ISP support and ask.

This if the previous answer of DHCP doesn’t work here.

And I’d expect Tesserax’s use-DHCP suggestion to work.

You're very welcome!

If you don't know your ISP-provided DNS servers, the simplest way to get them "back" is to remove any DNS server entries in your Time Capsule's settings.

To do so:

1. Run the AirPort Utility; select your TC, and then, select Edit.
2. Select the Internet tab. Verify that Connect Using is set to DHCP, and that there are no editable entries showing for the DNS Servers. If any entries are greyed-out, these would be coming from your ISP.
3. Select Update, and allow your TC to restart.