It looks as though your previously-installed version had an extra digit on its version number, making it higher than the new version (which doesn't have the extra digit). Moodle therefore sees the new version as "older" and gives you the error you mentioned.
If you run the following query:SELECT * FROM mdl_config_plugins WHERE plugin='assignsubmission_mahara' AND name='version';
you should get a single result. If so, you can then run:UPDATE mdl_config_plugins SET value='2014041200' WHERE plugin='assignsubmission_mahara' AND name='version';
which will remove the extra digit; the upgrade should then run without issue.
If you get more than one result from the first query, post the results here so we can figure out which one(s) you'll need to remove.