Assuming there aren’t any health issues I’d recommend you start exercising and then deal with your diet. Also, give it time to take effect.
You need to lift weights, get your cardiovascular fitness up, plus something for flexibility. It’s also nice to have one or two moderately active hobbies, even if you don’t do them all the time. Find things you enjoy and remember your body needs fuel to build muscle. It’s great to be slim, but as we age we need to build muscle, bone density, and flexibility.
At 50 I’ve realised that what worked for me in the past (just eating less, basically) just won’t work any more so I have to exercise and I’d rather be a tiny bit bulky (I’m currently slightly over BMI for the first time) than ‘skinny fat’. I do lots of exercise and if I don’t eat enough (especially protein) my energy levels really suffer. I’m realising I’m in this for the long haul and that’s good!
We need to be realistic and aim for healthy BMI but not chase numbers on the scale to the detriment of bone density, strength and good posture.
Look at some older people and see how dramatic the loss of muscle, balance/stability and strength can be. That’s the strength we need to do all our everyday things. Also exercise makes you feel better which IME is priceless in menopause!