In MoI with a basic cube you can turn on show points then select the 2 point of that edge and drag them around or invoke the move command and numerically move them around.

But this will be limited to the cube.

Its not that “basic instruction is missing”

Spaceclaim does whats called “direct modeling”

MoI is not a direct modeler, so not understanding there is a difference is the issue.

In MoI you would reshape your initial construction geometry or perform other operations to change the shape of the cube.