Sorry your keyboard needs to be replaced you don’t have a software issue.
Let’s look at how a keyboard works… Think how a spreadsheet is laid out in columns and rows. Here the column is at an angle starting from the top with the 7 key, then down to the U, J & M keys. So the electric line that serves these keys likely has rotted out within the keyboard proper. Over time the junk that was still left from the spillage rotted the line out.
The easiest way is just replace the uppercase [product|IF117-009-5] and here’s the guide [guide|9736]. Given the systems age you might as well replace the battery as well [product|IF117-047-5] as its just safer to replace than to try to reuse your battery. Here’s the guide for it [guide|89197].