Use sed to run replacements based on regular expressions.

# To replace the first occurrence of a string in a file, and print the result:
sed 's/find/replace/' filename

# To replace all occurrences of an extended regular expression in a file:
sed -r 's/regex/replace/g' filename

# To replace all occurrences of a string in a file, overwriting the file (i.e. in-place) (GNU):
sed -i 's/find/replace/g' filename

# To replace all occurrences of a string in a file, overwriting the file (i.e. in-place) (BSD):
sed -I '' 's/find/replace/g' filename

# To replace only on lines matching the line pattern:
sed '/line_pattern/s/find/replace/' filename

# To apply multiple find-replace expressions to a file:
sed -e 's/find/replace/' -e 's/find/replace/' filename

# To replace separator / by any other character not used in the find or replace patterns, e.g., #:
sed 's#find#replace#' filename