Presuming you are at home, if the brisket is still in the cooler, stick a temp probe in it. As long as the temp stays above 140-145, you are fine. If it drops below 140, take it out of the cooler and stick it in a warm (140-160 degrees) oven to hold. If your oven doesn't have that low of a setting, heat it to the lowest it will go then turn it off and put the brisket in.
Long story short, you don't want the brisket to fall below 140 as it's a food safety issue. On the other hand, you don't want to continue cooking the brisket at 225 either, as it's already done. So, do what you need to do to keep the brisket between 140 and 160 degrees while holding it to serve for dinner.