Fixing bugs on an existing project
First, thanks for stepping in to fix bugs in an existing project!
Find the project in the Studio Projects menu and request read-only access. If you cannot find it in Studio, open a support ticket to ask for access.
Once you have access to the Manage game page, you can read the code and try to find the cause of the bug.
Find the project administrators in the right-hand column. There are links to both their Studio and BGA accounts. Check their profiles to see if they have logged in recently or stopped using BGA or Studio.
Project with at least one active maintainer
If a project administrator is still active, contact them to see if you can work together on the bugs. Try their BGA account first, rather than their Studio account.
Depending on their interest in the project, they might want to fix the bug themselves, perhaps using your analysis. If they don't have time or don't want to handle it, they can give you admin access so you can fix it directly.
Project with no active maintainer
Open a support ticket to ask for access. Once you have access, you can fix the bug as described below.
When you have administrator access to the project
Test your fix and release the updated version, just as you would for your own project. See Start a real project for more details.
Conclusion
You've reached the end of the Get Started section. Congratulations, and welcome to the BGA game development team! You should now have everything you need to start working on Studio.
Remember to have a look at the Framework documentation while working on your project.