The U.S. Department of State says that crime in Mexico varies widely by location with the worst crime reported in the states that border the United States and the south of Mexico, mostly because of drug cartel activity.
Guanajuato has been a relatively safe city and crime generally is moderate. However, in the last three years, the crime rate has spiked overall in the state of Guanajuato. At this time, there is no safety advisory in effect for the city or the state. Expats should be able to travel anywhere within the area and feel safe.
You can stay updated on travel warnings and safety information at this U.S. Department of State website.