how can I fix a tripped breaker?
If it's an old A/C unit, replace it with new.  They are much more efficient (meaning: same BTU, much lower energy draw) and it'll pay for itself in energy savings.  I just saw a no-brand cheapie that was 8000 BTU for 5 amps.  The lower current draw means fewer or no breaker trips.  

Even an old A/C is unlikely to trip the breaker alone.  Most likely it is the "last straw" on an already loaded circuit.  Next time it trips, thoroughly survey what else is on that breaker.  Research how much current or power each of those loads takes.  The best way is with a power meter like a $20 Kill-a-Watt.  Measure peak loads while the appliance is "on", e.g. A microwave while it's cooking, a laser printer while it's printing. 

To be honest, most consumers have no clue how much energy appliances use.  This may be an eye opener.   The #1 surprise runs Microsoft Windows.  A lot of PCs are sold with 850 watt power supplies which can take as much as 1200 watts, though this depends on what the computer is doing.  The dead giveaway of a high-draw appliance is it makes a lot of heat.  

And that especially matters if you are trying to air condition the space, it creates two electrical loads: first to run the appliance and make the heat, and then to run the A/C harder to remove the heat.  Reducing one reduces the other.