There's not a single, one-size-fits-all solution, there's a lot of factors going into this

In no particular order, off the top of my head, a couple things that would need to change

1. Better nutrition/health education- a lot of people have almost no idea what eating healthy means
2. Better labeling requirements, there's a few weird loopholes in how food and such can be labeled, and companies get away with using some pretty misleading labeling, and we need to be tightening that down so that people can actually make better informed decisions about what they're eating.
3. Better pay and work/life balance- Planning and preparing a healthy diet takes time and money. Same with going to the gym. We need to free people up to be able to make those better, healthier choices for themselves
4. Addressing food deserts- In some places people just may not have easier access to healthy food. If you don't drive and public transit is spotty around you, and the only stores within walking distance of your home are gas stations and convenience stores, it's going to be hard to eat a healthy diet.
5. Pedestrian and bike friendly infrastructure- In a lot of cases, people don't walk or bike places because it's just not safe for them to do so. If you provide that opportunity for them, a lot of people will actually use it, they'll get more exercise, and save money on gas that can in turn be used for other healthier choices.
6. Healthcare- In some cases it may mean getting things like glp-1 drugs or surgeries to aid in weight loss, in other cases it might just be checking in with a doctor or nutritionist to come up with a weight loss plan to make sure you're going about it safely and effectively. Maybe you need blood work or other tests to identify other underlying health problems, maybe you have depression or anxiety that are making it difficult for you to be motivated to go to the gym or some sort of trauma you need to work through to address an eating disorder. Whatever the person's specific needs are, they need to be able to access them.