one of the most powerful commands I rely on is the substitute command. To replace text, you use the syntax :s/old_text/new_text/. For example, if I want to replace 'apple' with 'orange' in the current line, I type :s/apple/orange/. If I need to replace all occurrences in the entire file, I add the 'g' flag like this :%s/apple/orange/g. The '%' means apply to the whole file. For case-insensitive replacement, I use :%s/apple/orange/gi. Vim's substitution is incredibly flexible, allowing me to add confirmations with 'c' or target specific lines by specifying a range like :10,20s/apple/orange/g.