Thank you for your question on changing the bobbin on the sewing machine.
It will depend on the style and model that you have.
Some have a drop in bottom from the top and you remove a little plastic window to remove the bobbin from the bobbin case.
The other style is a front loading bobbin and you will have to open a door in front in the free arm and reach in and pull a little lever out to remove the bobbin case.
In the images below the top picture is the drop in bobbin type and the bottom picture is the vertical type bobbin.
Please reply with the model number of the sewing machine if you need more information.