You probably can't, but you can try.
If the issue has been rectified you hold the stop switch down for a few buttons to get rid of the blinking. If the issue still exists there is nothing you can do (short of correcting the issue) and it will return.
The Onan will hold the last code in stored memory. It won't be displayed once the issue is corrected but it will always be there, lurking in NVRAM someplace. If you really want to erase that history you can disconnect the battery cables from the genset for 5 minutes, then reconnect. Sometimes that works, but not always. It's model dependant. Regardless, if the issue has not been corrected it'll be back.
Mark & Leann Quasius
2016 Cornerstone 45A
2007 Allegro Bus 42QRP (Sold)
2012 Jeep Wrangler Unlimited - Rubicon