In the folder data/language you have text files for each language, if you want to change this text directly.
Alternativley you may change these strings in an additional ruleset, for example:
extraStrings:
- type: en-US
strings:
STR_PLASMA_BEAM_UFOPEDIA: Hey, with this gun you can shoot down everything!
![Wink ;)](https://openxcom.org/forum/Smileys/default/wink.gif)