there's no easy way of doing this but once you know what to look for it will become easy
If you don't know - learn how a template-based system works
Skinning: Understanding A Template-Based System
Read up on the many articles found here.
https://vborg.vbsupport.ru/forumdisplay.php?f=184
For now, I suggest having a "test" forum and do all your edits there.
If you are unsure of what templates are used, enable ADMINCP / vBulletin Options /
Add Template Name in HTML Comments. This option will add the template name to your HTML source.