In BCV there are two bytecode editors, one for android apks (smali) and another for Java classfiles/jars (krakatau).

Simply go into then tick on
View>Krakatau>Bytecode
Also tick on
View>Krakatau>Editable

Then to compile either go into File>Compile, or go into Settings>Compile On Refresh then click the 'Refresh' button at the bottom middle of the screen.

Refresh will compile, then update all of the panes you've toggled on with the new Bytecode if it was successful.

Quick note about CJBE/JBE:
They're outdated and buggy so classes tend to break very easily, I recommend using DirtyJOE or Krakatau Bytecode.