The shortcut Ctrl+PageUp is to move to the previous sheet in a workbook or stay in the first worksheet if the cursor is already in the first worksheet. Page Up key is also PgUp in some keyboards.
Step 1: Click anywhere in the worksheet (e.g., Sheet 2);
Step 2: Press and hold the "Ctrl" key, and click "PageUp" key. The cursor will move to the previous worksheet, which is Sheet 1 in this example.