Change One Value In A String

I want to check a string for a certain value then change that value to something else.
Example, user types in a value in a form "jim" , some code checks to see if the letter "m" is in the string. If it is it changes it to "t".

But we don't know if the string is jim everytime. Sometimes it could be "mark" or "quagmire". But for any string with letter "m" i want to change it to "t". So "mark" would become "tark", "quagmire" become "quatmire" Is this possible?