LaTeX 2.09 documents are characterized by using the \documentstyle statement to specify the base typesetting specification. LaTeX2e uses \documentclass for this purpose.
A quick hack to convert from LaTeX 2.09 to LaTeX2e is to use a text editor on the .tex file to change \documentstyle to \documentclass. This is not completely reliable as there are other aspects of a document that are different between the two formats.
A more reliable conversion method using SW 5.5 is to start a new document (all the Version 5.5 shells use LaTeX2e), and then either use File, Import Contents to select the previous document, or use copy/paste to transfer text from the old document into the new document.
The modified or saved .tex file can then be imported into Version 6.