Hi Alex,
for menu and categories of Joomla articles the priority can easily be set using the widget in the data source settings, see below screenshots, of course you find more info in the pdf guide.
In this way if a menu is linked to a category of Joomla articles you can easily change the priority for that menu and for all the related articles.
If the menu points to an article category of K2 the priority has to be set at data source scope, thus you should use different data sources to set a higher priority for those K2 articles of interest. For example one data source can include only K2 articles for a certain category with priority 100%, and another can exclude that category and having 50% priority as default for all other categories.
Regards,
John