It is called 'RECOMPUTE' or 'UPDATE' in Solid Edge.

1st things 1st. When you try to edit a dimension value and it results in an underlined value, it simply means the dimension is unable to change to the given value but reflected the modified value anyway in underlined form. This is due to conflicting dimensions or constraints from other elements.

To create a mounting boss, you'll 1st define a parallel plane with an offset from the target solid surface. Next you define the boss parameters from the options dialog and complete the command by specifying the arrow towards the solid surface. You can still later edit the boss parameters or positioning by using either 'EDIT DEFINITION' or 'DYNAMIC EDIT'.

A tool to use in 2D sketch environment to aid your constrainting techniques and dimensioning is the 'RELATIONSHIP ASSISTANT'. This tool can be found in sketch environment at,
TOOLS > DIMENSIONS > RELATIONSHIP ASSISTANT.

Hope this helps.