Refer to How to reset your AirPort base station - Apple Support.
The effect of a "hard reset" is such that the Time Capsule will disappear from the wireless network it either created, or joined. Instead, it creates a temporary wireless network that exists for the sole purpose of configuring it.
That means you will need to manually connect to that temporary network. To do that click the Wi-Fi icon in your Mac's menu bar:

... and select "Apple Network nnnnnn".
- This disconnects your Mac from your usual wireless network, meaning you will not be able to access the Internet until you select your usual wireless network again.
Then, launch AirPort Utility to configure the Time Capsule. If you need help doing that please write back. We will need to know what you want to do with the TC (create a wireless network, join the other Time Capsule's wireless network... etc).
To set up a TC that has been "hard reset" follow these instructions: Set up a new base station using AirPort Utility on Mac - Apple Support.