Changing interest is done in client information. In Timeslips 2016 it is on the Charges screen under "Bills section". In other versions it may be somewhere else. You can make the change and export to other matters but please make a backup first and be careful, as this is no undo.
Changing the administrative fee is harder to say as I don't know where they set it. If they are using sales tax for this, then that is where it would need to be changed. There are several ways to do the administrative fee so i can't be certain without seeing the database.

Thank you Caren. The current Admin fee is set at 2.5% and they want to raise it to 5%. The firm administrator mentioned that it only showed up to 3.5% in the program. They are using the Admin fee in place of charging for postage, paper, etc.

Are there laws to abide when it comes to interest rates (annually) and what you can charge? As in a cap of some sort? We are in California if that makes a difference. Always appreciate the help here!

I would need to look at your system to give you any additional information on the admin fee.
With regards to interest rates, if you are a law firm, check with your bar association. If you are not a law firm I would check with your state consumer protection department.