I like doing this in terminal, when major updates come up, you should always come here first, look at the page for your branch (Testing is here - on that page you will see information - and warnings (and always a link for the Mirror-Check Service
To do a ‘Mint’ style run, you can do `pacman-mirrors’ and they all get tested - while you wait…
Best start this when you just got coffee…
When finished, it orders it’s results and offers you a GUI to click some checkboxes.
I recently decided to switch branches and came up with one or two new alias commands which do the job well, and quickly. However, after Mint - you’ll feel happier with the LONG method I guess 
alias mirror='pacman-mirrors' # list MY mirrors
alias mirror-default='https://wiki.manjaro.org/index.php?title=Pacman-mirrors' #also returns to STABLE branch
alias mirror-fast='sudo pacman-mirrors --fasttrack 20 && sudo pacman -Syyu'
alias mirrors-choose='sudo pacman-mirrors --interactive && sudo pacman -Syyu'
alias setbranch='sudo pacman-mirrors --api --set-branch ' #stable/testing/unstable