If I have a Number in Cell A1 and then want to add say 10 to that number, I
then want to replace the Number in A1 with the new results.

If A1 has 10
Then A2 has 20

I want A1 to then say 30.

If I change A2 to then 10 again
Then A1 changes to 40.

I need like a running SUM in a Cell.