Depends where on Hanover. The stretch from Elm to Chestnut is very nice and tree lined. The area around Bronstein Park and Bagel Cafe is run down but not dangerous. Once you get past the Hanover/Belmont St intersection it gets nice again. Colorado Springs has worse crime than Manchester on average so as long as you have city smarts you'll be fine. Lock your car doors, don't leave any bags or valuables visible in your car (including loose change), etc.

The r/newhampshire subreddit and online forums project a comically negative image of Manchester. It's laughable to see people comparing it to South Chicago whenever we have a murder. Most people who criticize the city don't even live here or visit very often. It would be naive to say this city doesn't have its issues (mostly related to drugs and the opioid crisis), but there is no better judge of safety than your own experience and we all come from different walks of life and have different expectations. By NH standards Manchester is terrible. I don't blame some people for thinking this way as many locals I've met here haven't even left New England (a testament to our high quality of life and opportunities). Please take the time to explore and enjoy our fair city. We are rich in history and things to do, plus the location in New England can't be beat for recreation.