That's a complex topic. Read the manual
https://wiki.librecad.org/index.php/LibreCAD_users_Manual#Dimensions and
https://librecad.readthedocs.io/en/latest/guides/annotate.html#dimensioningBe sure to have free snap enabled to be able to place the dimension where you want it. When you select an existing dimension you can pull the value text to desired position by the blue square.
Experiment with the dimension settings in Options - Current Drawing Preferences.
You may provide a sample drawing here so I can have a look at the settings.