Members with Administrator permission or the Account owner can edit Administrator permissions. Simply:
Click on the Organisation icon on the left-hand blue navigation bar.
In the People tab, select the team member(s) you wish to update permissions for by checking the box to the left of their name.
Click on the 3 ellipses to their right of their name and select Permissions from the drop down menu.
Move the toggle across to enable Administrator or Budget permissions.
TIP: You can only grant Budget permissions if you have Budget permissions yourself or are the Account Owner.