It depends on what you mean by "update an existing mind map" and also what version of MindManager you have.
If you are trying to update selected existing topics spread through a map using information in a Word document the short answer is no.
If you want to attach a Word document or portions of a Word document to a topic in an existing mind map the answer depends on the version of MindManager you have. Up until version 19 or 20 this was possible because MindManager installed a Send to MindManager add-in in Word which allowed users to do exactly that. This was a great feature which I used to use extensively, but the Word export facility in MindManager was completely rewritten in later versions. This improved the export process but for reasons I still don't fully understand required the "deprecation" (aka "removal") of the Send to MindManager feature.
Without this facility your options are limited. The only way now to import a Word document is to import the whole document from the MindManager end into a new mind map. If you want to attach part of a Word document to an existing map, the only workaround that I know of that works directly with Word is to copy the required section of the Word document to a new file, import that file to a new mind map, then copy the contents of that map to the relevant topic in an existing map.
If the Word document happens to be a table - or easily convertible into a table - you may be luckier, though it depends on the nature of the data. You have the option of copying the table into an Excel-compatible spreadsheet and then importing it using the Excel Data Mapper. Unlike Word import, this allows users to select a number of options about how the imported Excel data will be formatted. This also includes the ability to attach the imported data directly to a topic in an existing map.