Assuming your default units already are in inches, otherwise change the drawing to the units you will be using before any measurement.
measure a line with known measures, If I understand correctly, you say countertop should be 24" but it currently measures 38"
select the entire drawing entities, then scale
select a base point anywhere it makes sense, it is not so important as you scale everything.
In the popup dialog set the scale to 24/measured_distance
A little late to the party. As a Civil Engineer, I am used to scaling up/down as elements outside of the building envelope may require. I measure an element in the drawing (say - a line, maybe a property line - or the width of a desk or interior wall if inside the building envelope), one that is long, and one that has a known dimension. If the element measured is too long (say), I divide the smaller known dimension by the larger measured dimension to establish the percentage of scale reduction required. Then I command:
SCALE==>(Select to scale)
SELECT ALL [button]==>
right click again in the drawing==>
SCALE==>(Select reference point)
0,0 [or other appropriate reference point]==>
[load the calculated percentage into the ISOTROPIC SCALE menu]==>
Save Early - Save Often, in particular immediately prior to SCALE.
Warning: Some blocks do not react well to scaling; dimension arrowhead blocks from AutoCAD in particular.
UNDO (after SCALE) doesn't get it done for me. Is there a presetting for the amount of allowable buffer?
When a commands "blow up" unexpectedly (I am new to LibreCAD), I close and reload the drawing, then try something else.