There are many different pieces of functionality in play with the Create/Edit Virtual Machine form
When you select the Create option to Create a Virtual Machine, you'll first be taken to a Template Selection page where you need to select a template. This can be changed later should you wish to do so.
Once a template has been selected, you'll be shown the main Create Virtual Machines form.
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/Billing Plan. It will adjust the value automatically if the value is too low or too high.