When you modify a dimensioned geometry, the attached associative dimension should be updated automatically. Verify this functionality on a newly drawn and dimensioned object.
If the dimension is still not updated automatically updated, try to enter the following command (LISP function) in your AutoCAD command line:
This will re-enable associativity. This function is rarely used, but temporary suppression of automatic updates may be a part of some add-on LISP utilities (makes them faster). They disable automatic updates with the function format (acdimenableupdate nil).
Even in a disabled state, dimensions can be updated manually, with the DIMREGEN command.