Replacing the main breaker isn't a DIY project, it's best left to a licensed pro. It will involve disconnecting the power to the house, and that means your local electric utility will need to get involved.
As for adding more circuits, it depends on the panel itself. It may be the bottom 5 busbars are designed to take half-size, or piggy-back, breakers. Look at the panel door to see if it lists a maximum number of circuits. If you have a 20-space panel, and the label says you can have 30 circuits, then it's permissable to add the piggy-back breakers.
Another option would be to remove a 240-volt circuit from this panel, and use the space to feed a small sub-panel next to this one. Relocate the old circuit into the new panel.