How to link a cell to another cell in two different sheets?

Making a link between two cells in two different sheets

You link a cell by referencing its address

=Sheet1!a1

Will link the cell you place that formula in to cell A! In Sheet1.

You link a cell by referencing its address

=Sheet1!A1

Will link the cell you place that formula in to cell A! in Sheet1.

... so depending on what you meant by "two" sheets, refer to each sheet individually.

If you want to subtract Sheet2!a1 from sheet1!A1

=Sheet2!A1 - Sheet1!A1

Beyond simple addition and subtraction, it gets tricky. If you provide an example of what you mean by "linking" (what do you want these links to accomplish?) then perhaps we can help more specifically.

It is the manually input month that I want to update between several sheets by just updating the first

That didn't work, it just placed that formula in the cell, does it make a diffence if it isn't numbers

If you did it right it should display what is in the cell you referenced.