Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the Time Capsule icon, then click Edit in the small box that appears
Click the Wireless tab at the top of the window
Click the Wireless Options button near the bottom of the window
Enter a check mark in the box next to 5 GHz name
Click Update at the lower right of the window and allow a full minute for the Time Capsule to restart
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