my mom won’t clean my beta fish’s tank and i don’t know what to do

So this has been going on for way too long my beta fish has a pretty decent sized tank, but it’s gotten dirty and the water is dark green and it’s really hard to see in. I asked my mom why she hasn’t cleaned it and she said she was afraid of killing the fish because our previous fish passed after a tank clean, and I don’t really know what went wrong, I never got told. The reason I haven’t done it myself is because i have no idea how to, i’ve never done it before and i don’t want to scree it up. we’ve had different types of fish all through my childhood (i’m 17) so I don’t know why this is an issue. i’m really worried for my fish because I know he can’t be happy in that filth.

I’ve tried to convince my mom so many times I don’t even know what to say anymore. If anyone knows anything i can try or say to her please let me know i am desperate to save my fish