You can improve a bad credit score. If something specific caused it, like a repossession or bankruptcy, it will take several years, It will also require negotiating repayment of some of what is owed.

In general, keeping a low debt to credit ratio, having only one or two credit cards, paying more than the minimum due on bills, and paying your bills on time will increase a bad score.

Yes, it can be fixed in exactly the same way a GPA can. That is, GOOD history will eventually outweigh bad history. There are no quick fixes, aside form disputing FALSE information.

Only with time and good credit behavior.