Using a workbook with VBA to open a saved workbook, modify that pre-existing workbook, save and close

Using a workbook with VBA to open a saved workbook, modify that
pre-existing workbook, save and close

I am fairly novice to VBA and am looking for some help. I would like to be
able to run a VBA command in an excel workbook that will call open a
saved, existing workbook, at a specified location (Desktop\ThisBook.xlsx).
After opening the ThisBook.xlsx, the command will take numerical values
from specified cells in the VBA workbok (A1), add them an existing cell in
ThisBook.xlsx (something like ThisBook.xlsx A1 = ThisBook.xlsx A1 +
ActiveDocument A1) and save them where the ThisBook.xlsx data was.
Finally, overwrite the previous save file for ThisBook.xlsx and overwrite
the new data.
Thanks in advance for any help or direction.