I have not been to Guanajuato myself, although my husband & I took an extensive road trip through several of the states in Mexico in the 1990's 2001. Guanajuato is a very large city, so just be careful of what parts of the city are safer and if you rent a car be very careful to obey the traffic signs. If you don't have to, I would advise against a car rental, in fact as I find walking and public transportation in large cities is more fun and usually easier than finding my way around in a car in a city with which I am not familiar even with GPS.
Get to know as much as you can about the transit systems available to you, what routes to take and utilize whatever mapping program you have available on your mobile device if you use one. But also keep close tabs on your devices as you walk around, maybe a wrist strap and ear plugs to hear directions.
Will you be alone or will you have a companion for the few extra days you will be staying? I travel alone a lot myself, but have never done so outside my own country, so don't have much advise about being solo in Mexico. My daughter-in-law was with us when we traveled to some remote areas, which turned out to be very good for us. We were stopped by a group of militia, serched and questioned. Following my DIL's advice, anything I did not want them to search or confiscate, I wore on my person because she told me they would not touch a woman or anything the woman wore. So my camera, money, etc. was in a bag I could don quickly.
Oh, one other thing, use travelers cheques whenever possible. And keep money in two separate locations on your person, one for larger bills and one for smaller bills. We had to pay a bribe to get out of being taken to jail in Mexico City, and I used my money purse with only small bills (US $1.00 bills) and travelers cheques; he wanted the whole wad of money, but it wasn't a lot, so I'm glad it wasn't $100.00 bills in that wad. My husband still brags about how smart I was to carry two money purses. One money purse in my bag, the other well hidden.
Just so you know, it wasn't a big deal. My husband accidentally turned into a one-way street and immediately turned back around. But in spite of the fact that it was obviously accidental and immediately remedied, a cop stopped us and threatened to take us to jail if we didn't give him a large sum of money immediately. I told him that was all I had except for traverler's cheques, so he took the whole wad of money that he saw me pull out. Travelers cheques can be cancelled and replaced, so that's probably why he wouldn't take them. Anyway, you can see why I advise against renting a car if you don't need one.