how would i replace a word or part of a word in a label containing a few paragraphs or text

I would like to be able to replace word, or parts of words, submitted in a text box before uploading.  How it works now, is they type in the information and hit submit, in a label below the TextBox, it shows what they typed, and how it will look. Then a button become visible for them to continue.  What i would like to do is parse through what they typed and replace or delete unexceptionable words or parts of word.  I know this can be done, but don't know how.

Thanks.

BTW  It's in  c#