My inner critic used to be incredibly cruel. Sometimes it would chant short negative phrases repetitively. It would regularly insult and belittle me. I used to physically self harm, but this was like a constant drip drip of emotional self harm, if that makes sense.
But now, the vast majority of time, my internal voice is kind, caring and loving. It took a lot of conscious work to make this change. For example, whenever I thought a negative thing about myself, I forced myself to think of three positive things. These positive things could be really tiny things and I tried to focus on facts so that my inner critic couldn't argue too much with the kind voice I was trying to cultivate e.g. I messaged my friend when she was feeling upset, I got out of bed this morning even though I wasn't feeling great. I was suffering from severe depression at the time so this was something I did multiple times a day, they were very regular negative thoughts. (But the negative thoughts predated the depression by years, so it wasn't as simple as the negative thoughts went away when the depression did). I also focused on trying to talk to myself the way I would do to a child or an animal. I realise this sounds ridiculous, but it worked. We tell children and our pets how wonderful they are and how much we love them. I decided to start telling myself how wonderful I was and using soothing kind tones. Yes, I'm an adult woman, but my relationship with myself became a nurturing one and it's made my life a lot happier.
For most people I'd imagine that a sustained conscious effort could turn a harsh inner critic into a softer caring voice.