While you are free to make any changes you see fit to all letters you create within UGotiate, it is recommended to leave the language as is within the offer letter. You should, however, take a moment to make sure your personal information (name, address, offer amount, etc) is correct within the letter.
The offer letter within UGotiate contains specific language that has been carefully written to hopefully produce a response from the creditor. There is also text directing the creditor what to do should they accept your offer. For example, there are certain items that must be on a settlement letter a creditor sends you for the letter to be considered valid and safe to pay. If you were to change this text, this might cause confusion when the collector reads the letter and/or delays in the creditor responding. Leaving the text as is, is in your best interest.