Changing a category of a purchase invoice

all the answers I’ve seen refer to doing a bulk change in the chart of accounts page - where is this page please?

It’s actually in the ledger detail page, I.e. when you click the blue magnifying glass icon on the Chart of Accounts page. It’s explained in more detail here.

Bulk change incorrect postings

If it’s a single purchase and it isn’t locked you could change it just by modifying the purchase directly.