Similar to the Create Virtual Machines Form, the Editing Virtual Machines form also has quite a few different components at play.
You can use the Slider to change the Resource Allocations. Alternatively, if you tap on the numerical value alongside the Slider, a number keyboard will pop up and you can manually enter a resource amount.
If you manually type a resource amount, it will be checked to see if it is within the boundaries of your OnApp Account/Bucket. It will adjust the value automatically if the value is too low or too high.