Change the measures of a box

[Christin83]

Dear all,

I would like to change the measures of the box without starting from the beginning again so to speak make the box less „deep“.
Is there a way I can do this? When I just change the extrude it cuts the bottom of the box

Many thanks in advance!!!
+

[TigerMike]

Here I have a wall thickness of .6mm. I want to shorten the depth by 2mm and maintaining the .6mm wall thickness.