Flicking between to sheets

Hi Ousmane, thank you for your post, welcome to the forum, in answer to your question; in the heading you say sheets, but in the question books, so I will answer both.
1. To move between worksheets, (in the same workbook) use the CTRL+Page Down or CTRL+Page up key combination, or use the mouse to click on the sheet name.
2. To move between workbooks, if both workbooks are open, you could click on the workbook on the Task bar where it is shown as a button, or use the Menu command Window> and choose the required Workbook. I hope that has answered your question, note not a mention of ALT+TAB, best regards Pete