You can reduce muscle mass by eating less calories than you need to maintain your weight, and simply exercising on top of it! Any exercise will strengthen the muscles used to some degree - just pick something you enjoy, like running or swimming or whatever that exercise is, do it, and stick to a good diet. If you are unsure of the calories required to lose weight/muscle, there are tons of online calculators to help you. Search for TDEE calculator and take 500 calories off the result as a good starting point.

One thing to note is I'm sure hormone therapy would have an effect on weight loss/metabolism, but I'm personally not exactly sure how you should approach that. If you aren't on hormones yet there's nothing to worry about in that sense though.

Also any form of resistance training, be it with high repetitions or not will cause some small degree of muscle growth. It shouldn't be avoided for that reason alone, as it would be impossible to gain much - if any muscle while on a diet (and resistance training is better for burning calories in the long run) but if you have a "significant" amount of muscle mass already it might not be such a good idea.

Best of luck :)