[tor-relays] How to manually change overloaded Guard?

Since tonight I can't get any usable Tor connections anymore; restarting Tor gives the following error message:

Guard TOR2DFNrelB ($0ED0EA324C931CF41CB5272BFB1D015B3D5772A9) is failing more circuits than usual. Most likely this means the Tor network is overloaded. Success counts are 152/217. Use counts are 45/59. 167 circuits completed, 13 were unusable, 2 collapsed, and 51 timed out. For reference, your timeout cutoff is 60 seconds.

Any idea how to change the Guard - just restarting Tor doesn't help?