To download and install these updates, head to Settings > General > Software update. Make sure under the beta updates section, the [insert device OS here] developer beta is selected. Then the newest beta should pop up for you to download and install.
The public version of the 8th beta should be released tomorrow. The public version of iOS 18.1 beta will most likely be released after iOS 18 is officially released to the public, sometime after the September Apple event on the 9th of September.