StringReplace["string", s->sp] or StringReplace["string", {s1->sp1s2->sp2}]
replace the string si by spi for all occurances in "string".
StringReplace["string", srulesn]
only perform the first n replacements.
StringReplace[{"string1“, ”string2", ...}, srules]
perform replacements on a list of strings

StringReplace replaces all occurances of one substring with another:

Multiple replacements can be supplied:

Only replace the first 2 occurances:

StringReplace acts on lists of strings too: