It is the nature of contracts that their terms can be varied. This letter template has been designed for use in relation to proposed contractual amendments, where the proposal is made by one of the contracting parties and is sent to the other contracting party (or parties).
All but the shortest professionally-drafted contracts include an express provision dealing with the method of varying or amending the contract. Often, this simply says "The contract may not be varied except in a written document signed by or on behalf of each party" (or equivalent). However, in some cases more detailed variation clauses are included. For instance, a contract may provide for a specific procedure for varying some or all of the contract terms - sometimes called a "change control" procedure (not to be confused with "change of control" procedures).
In any case, before proposing a variation of contract, you should review the contract to determine what, if any, procedures you need to follow to ensure the variation is effective.
In the letter template, three different types of contractual amendment are highlighted: additions, deletions and substitutions.
Note that in the event that amendments are extensive, it is usually preferable to set these out in a version of the contract showing tracked changes, rather than in a distinct letter. Even if you choose to send a letter, you may wish to provide a copy of the contract with the relevant changes highlighted.
The template letter provides that, if the recipient accepts the proposed amendments, a copy of the letter should be signed and returned to the sender within a defined period following the date of the letter.