# Replace all instances of foo with bar
:%s/foo/bar/g
# Replace all instances of foo in lines 6 to 10 inclusive with bar
:6,10s/foo/bar/g
Move the cursor to the appropriate location.
Enter 'VISUAL BLOCK' mode (Ctrl v).
Use your directional keys to select required number of lines (j or k).
Press I (capital i).
Press Esc.
Note: Once exited out of 'VISUAL BLOCK' mode, changes will take effect.
:5,12s/foo/bar/g