Hi Carol, it does look like it is time to repot.
@Bonsai Nut told you right about technique to change out media. You will be able to tell what type of soil it is in when you take it out of the pot.
To prune? Or not to prune? Really depends on what you want? What do you want it to grow into? How thick a trunk do you want?
It might help to do a sketch, and make some notes about your plan.
If you want a trunk thicker than your thumb, you have some growing to do. A bigger training pot would be the way to go. Maybe a 6 inch or 9 inch diameter bulb pan. If you like the trunk the way it is, you can put it back into the same pot.
Just looking at it, you have 2 branches that are as thick as the trunk. And they are at 90 degree angles to the trunk.
If you want to keep it small, and keep it in the bonsai pot it is currently in, and pretty much want to keep it as is, I would prune all the branches to just one or two leaves each. Then let it grow out again.
If it were mine, I would repot to a 6 or 9 inch bulb pan. When I repot change the angle of the trunk to roughly a 45 degree angle, and put the second up from the roots thick branch on top, going up at roughly a 45 degree angle. The branch will become the extension of your trunk. The next segment of the trunk. cut off the first thick branch. Cut all the fine branches to just one or two leaves. Shorten the new "second segment" of the trunk to roughly 2/3 the length from the soil to the end of the first segment. You will be building a tree, one segment at a time. Then all summer let it grow out. Every year or two years you repeat the chop it back process. Each time adding another segment to the trunk.
But do answer the question as to what you want. I would shoot for an upright tree as in my proposal. But Natal plum do make good cascades if you like. Or any style in between. So think about what you want. make notes, and then you will be able to head there.