The first time you upload a sie4 file, click on the "Economy" in the left menu and click on “Select file”.
To upload more files afterward, click on the "Economy" in the left menu. Thereafter, click on the "Data Management" button in the upper right corner. There you will find a button at the top right with the text "Upload SIE4-file", click on it to upload more files.
Do not limit the data to a specific month when exporting. Instead, include all data for each financial year, including the current one. It does not matter that there is overlapping data during the year, the system is based on the most recently updated information in the files. The files that are not used will become gray in the graph.