Use a spanner on the 'nut' fittings to unscrew. They will be a little tight - too tight to unscrew with you bare hand, but will come away with ease by spanner.
Note the white stuff on the threads, it's a thread locker/sealant.
Clean it away from all of the threads when you have removed the fittings. If you are replacing with new fittings they will probably be pre-treated but if the threads are bare or you are refitting the same parts then add either some loctite thread lock/sealant or ptfe tape.
If you use loctite then there is no need for high strength - 242 or 222 will do. If you use a high strength then you will find it hard to remove again and will have to heat to break the seal.