One of the basics of Excel is that I can define a cell's contents by creating
a formula in that cell that links to another cell. For example, I can enter
the formula =A5 into cell A1 and A1 would then be equal to whatever is in A5.

This works great. However, I would like to do the opposite. I would like to
define another cell's contents by creating a formula in the current cell.
For example, I would set cell A5 equal to say "nnn" by entering a formula
into cell A1. That is, cell A1 would modify an external cell A5.

Is this possible using Excel's basic formulas or would I have to use VBA?
Moreover, can I automatically run a VBA macro from a cell, in pretty much the
same way that say the formula =SUM() automatically runs?

I would appreciate any advice. Thanks.