is there somewhere clean to get changed and freshen up in Hangzhou Airport?

Just changing clothing or freshening up, U can use the toilets of the airport. They are clean enough.

If you also wanna take a shower, the airport has VIP lounges operated by different airlines. They may have shower facilities. But they seem only to service the first class and business class ticket holders. If you happen to have economy class ticket, the Airport Hotel shall be an option. Free shuttle buses can take you from the terminal building to the hotel in 3min. There you can clean and freshen up as well as have a rest.