Unfortunately Outlook does not let you change the ordering of folders, instead insisting on an alpha-numeric order.
There are two possible workarounds:
Rename the folders, putting additional characters at the front of the name to achieve the order you wish (e.g. 1 First folder, 2 Second folder)
Add folders to your ‘Favorites’ list which can be ordered.
To add a folder to your Favorites list and change the ordering
- Right click the folder you want to add and select ‘Show in Favorites’
- To change the ordering right click any folder in your Favorites list and use the ‘Move Up’ or ‘Move Down’ buttons