I want to plug in a refrigerator in a garage space which is on a 15 amp circuit. Can the breaker switch be replaced with a 20 amp breaker to support the extra load?

No, not unless you upgrade the wiring to 20 amps (12 gauge) also. The 15 amp breaker protects the wiring which can only carry 15 amps. The refrigerator pulls less than 15 amps. What else is on the garage circuit?

Thanks for your advice. To my knowledge, the only other device on the circuit is a power unit for the Web phone/cable and perhaps the garage door opener. The refrigerator pops the circuit only when it has to run a lot, i.e. when the garage heats up in our 100-degree weather. My temporary remedy is to run a power cord to a 20 amp outlet in the utility room.

Check refrigerator door for amps. Should be much less than 15. Don't know what opener pulls. Could you get a new cable from outlet in utility room to garage? Would be inclined to believe breaker is tripping when refrigerator is running and opener is used. 100 degrees just means that refrigerator is running more, increasing the chance of that that it is running when opener is used

Could it be that the breaker is weakened somewhat?