When you click on one partition to extend, there are 3 ways to extend partition:
Use the handle on the top, which has two directions, rightwards to enlarge space, and leftwards to reduce space.
When you drag the handle, Extender will first check if there's unallocated space behind, if not any, Extender will extend target partition with free space from the partition right next to it. See the handle below:
Input you specific amount into the box. New size/ Increased Size in MB
New Size: you can type in new partition size directly to extend Increased Size: you can tell Extender how much amount you want to extend,
Then OK to confirm.
Click the flashing right-wards arrow to do maximum extension.
When you click on NEXT, this windows will pop-up, and the Right arrow will flash to guide you quick extend.
Maximum size extension will make target partition maximum size, while the partition right to it minimum size.