To reset the drive I’m assuming cleaning off the data and prep it to be reused.
First step is to create if you haven’t already a bootable macOS drive with the desired OS version (supported by the system) Mojave macOS 14.x reference this doc on how to get the needed OS installer file [https://support.apple.com/en-us/102662|How to download and install macOS] then using a 32GB USB thumb drive copy the file to it (make sure to format the drive with GUID and a journaled file system). The next step is converting the file into a bootable state, follow this guide [https://support.apple.com/en-us/101578|Create a bootable installer for macOS].
Now with this tool you can boot up your system with its new drive f it’s not setup yet. If your external drive is what you want to clean off just run the format option making sure you select the correct drive. Within the format options is a security wipe which you might want to use.