When training calisthenics and moving your body weight through space you can’t just “change” your body weight.
But what you can do to vary the level of difficulty is changing the leverage, in the case of the back lever by changing the leg position.
Get into a high incline backlever position and then tuck your legs in. The closer you tuck the legs to your chest, the easier it will be to perform the exercise. While keeping your core tight and contracting your upper body, slowly descend closer to a horizontal position. Don’t just fall down into it, move only as far as you can while controlling your body position. The goal is to eventually be in a fully extended back lever, so once you can hold the tuck backlever horizontally start gradually extend your legs at the next try!
If possible, ask a training partner to give you feedback about the the angle of your body, since judging whether you’re horizontal or not by yourself is very hard at first, you might feel like superman and think you’re fully horizontal, while you’re actually still at 45 degrees!
Remember two things:
1. The more you tuck you’re legs, the easier it is
2. Always keep your core and upper body tight, controlling the descent into the horizontal position!!