Convert varchar column to datetime
Assuming dates in other months are truncated to 3-char months:

 ALTER TABLE dbo.tablename ADD newcol DATETIME;

 UPDATE dbo.tablename
   SET newcol = CONVERT(DATETIME, 
    REPLACE(LEFT(varchar_col, 11), '/', ' ') 
        + ' ' + RIGHT(varchar_col, 8), 113);


Once you've verified, you can drop the varchar_col, rename newcol, and then re-add any affected indexes.