You need to sort out if you're wanting to replace a char or a string. The letter 'b' is different than the word "ababa", so you should be a little more clear there: are you wanting to replace 'b' with 'c' or "ababa" with "acaca"?
Regardless, you're going to want to use the
UPDATE your_table SET your_field=(REPLACE(your_field,'old-text','new-text')
is (are) the character(s) you wish to replace, and
is (are) the character(s) you'll replace them with.