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# to search for the first occurrence of the string ‘foo’ in the current line and replace it with ‘bar’
:s/foo/bar/
# replace 'foo' with 'bar' in the entire file
:%s/foo/bar/g
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To quickly change a word you can use cw,caw or ciw. Use c$ or just C to quickly change from the cursor to the end of a line, cc to change an entire line, or cis for a sentence