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what do you mean by you can't edit? please describe the problem in detail, what you did, expected result, actual result?
A problem with the wysiwyg editor?
are there any script errors on the page in the javscript console of the web browser?
if you are getting script errors on the page, most problems can be solved by applying the needed changes from Important Skin Changes
or you could also copy a newer skin from /Data/skins/ into /Data/Sites/[SiteID]/skins and see if that solves the problem using a newer skin.
are you using CKeditor or TinyMCE? or maybe you were using FCKeditor since you upgraded from a very old version, if so go into site settings and choose ckeditor instead.
Hope that helps,