I have 2 sheets.Sheet 2 cell E5 has a number 123 I want to link it to sheet 1 A4 but when I get to A5 it must be updated automatically to E5 sheet 2:mad:

On sheet 1 in cell A4, put this: =Sheet2!E5

Now anything on sheet 2 in cell E5 will show on sheet 1 in cell A4. Automatically.

Your second comment makes me think you want anything you type in cell A4 to transfer automaticaly BACK to Sheet2!E5, and that's not possible with basic formulas. References go one way or the other. If you want what you type on Sheet 1 to be the one in charge, then reverse what I've suggested and put =Sheet1!A4 on sheet 2 cell E5 and now it will reference.

To have two cells equally transfer input back and forth regadless of which cell you change COULD be accomplished with a VBS script, but that's opening a can of worms you may not be ready for.