HI tdpascosa,
First, I assume you're talking about the breaker on the front of the generator. You may have to contact Onan to verify, but it's likely one of
these. You may need the model #, serial #, and spec level of the generator.
Second, if the problem you're trying to solve is that you're running multiple A/C units, or some other loads that use the entire capability of the generator, such that plugging in a very low power phone charger trips the breaker, the answer may depend on what you're running, as much as whether you change to a 30 amp breaker.
An Onan 5500 generator supplies a maximum of 5500 watts. Yes you can have two 30 amp breakers, but that doesn't mean you can pull 3,600 watts (30 amps @ 120V) on each breaker for a total of 7,200 watts.
And if you're tripping the breaker without putting much of a load on the circuit, something else is wrong.