After installing the Microsoft Apps I can open excel files, but I cannot edit nor do any changes on them. How can I fix this?

Microsoft Apps Excel supports only viewing for the old format of excel [97-2003] files. You can still use Quickoffice to modify those excel [.xls] files by first starting the Quickoffice application and then opening the excel file.

You can set Quickoffice to be the default application to open the old excel files with the following steps:

Select  Menu > Settings > Application Settings > Default Applications > Options (icon on the lower right corner) > Advanced options > Open application > Application > Quicksheet.

Or alternatively load the files on a PC save it in the new format, then you can edit the excel file.