I have the App dropbox for photos can anyone suggest how I can transfer back up to my MacBook using and apps I am upgrading my iPhone so don’t want to lose them .
If it is in dropbox, it should be in the "cloud" right? So regardless of what device you put dropbox on, it should have those files available.
Your question is a little difficult to read - either that or I haven't had my coffee yet (most likely the later).
As long as each photo is in Dropbox (meaning you can log into https://www.dropbox.com and see every photo there) then the next device you get, you can access all that data.
Highly recommend not having one point of backup. Open iTunes and do an iTunes backup. Purchase 50GB iCloud storage so you can upload your photos to iCloud Photo (or $1.99 for 200GB). It will take some time to do this.
iCloud Backup also backs up your photos if you don't have iCloud Photos turned on.
Dropbox will take all your photos and put them in Drobpox too.