Below is a worked example:
The data set I used is auto.dta. The commands I used in a do-file is:
sysuse auto, clear graph bar (mean) price turn mpg weight, over(rep78) set obs 75 replace rep78=2.5 in 75 graph bar (mean) price turn mpg weight, over(rep78, relabel( 3 " " ))
If you would like to know more on Stata graphs, you will find the book A Visual Guide to Stata Graphics, 3rd Edition very useful.