Singing lessons, or a choir with a good director.
Or expand it yourself, from the natural and relaxed reach you have in your range, by practicing scales. Practice that from the relaxed mode of singing. Slowly your reach may expand.
When I started studying I joined a choir with a very good director. We sang lots of scales. I started as a tenor, after a year I could do mezzo-soprano parties too.
Then I moved, joined another choir, bad director, working from stress rather than relaxed singing. Took singing lessons, teacher worked from tension. My range dropped massively. The joy was gone from singing. A friend of mine had a voice good enough for professional singing. She had a teacher that worked from relaxation. Did sing a bit with her. But her health was very poorly, so it never became a regular thing.
Now find myself singing a bit more, and my range is expanding a bit.
So, singing scales and have fun at singing. Working your power from a relaxed core, similar to martial arts, if that makes sense, is what I would advise. Including exercises to relax shoulders and whole body, before singing.