The easiest method is to copy the contents of the text file back into the Global Config mode of your switch.  I've copied the instructions with the required commands below.  More can be found at the link at the bottom including ways to setup a TFTP server to backup and restore configurations.

RESTORE A CONFIGURATION

1. Connect to the switch/router that needs the configuration.

2. Open the config.txt file.

3. Highlight the entire contents of the config.txt file.

   You can do this by dragging the cursor from before the first character to after the last character in the file while holding down the left mouse button. Alternatively, if you use Notepad, you can choose Edit > Select All from the menu.

4. Copy the selected text to the Windows clipboard.

   You can either choose Edit > Copy from the text editor menu, or hold down the CTRL key and simultaneously press the Ckey in order to perform the copy.

5. Switch to the HyperTerminal window, and issue the configure terminal command at the Router# prompt. Then press Enter.

6. Paste the configuration file into the router by selecting Edit > Paste to Host on the HyperTerminal menu.

7. After the configuration has finished pasting and the router brings you back to the configuration prompt, issue the copy running-config startup-config command in order to write the configuration into memory.

8. Issue the exit command in order to return to the Router# prompt.

Reference:

http://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-software-releases-122-mainline/46741-backup-config.html