The 'Amending' refers to the modifications carried out to the terms and conditions (for instance changing the New Expiry Date, New Expiry type, New Liability type, Amount Increase/Decrease, Predefined guarantee text can be chosen, and changes based on the selection criteria of the 'Choice of Text' and other related amendments) of an existing Guarantee contract.

The Amendment panel comprises all the fields contained in a SWIFT amendment message. Any changes made on the Amendment panel automatically update the corresponding field in the contract.

To Amend an existing open Guarantee contract (Guarantees and Standby L/Cs), GITAME transaction is used.  

Select it from the menu item [Guarantee Products -> Guarantees/Standbys -> Amendment-> Amendment]. 

(Users can directly start this transaction by clicking the (Launch transaction ) available in the Panel headline without having to select the relevant menu) 

Similar to purpose codes defined for Issuance of Guarantee, following codes are used for amendments respectively.

• ISUA – Issuance of amendment to issued undertaking
• ISCA – Issuance of amendment to issued counter-undertaking and/or request to issue amendment to local undertaking
• ICCA – Issuance of amendment to issued counter counter-undertaking and/or request to issue amendment to counter-undertaking and local undertaking
• ACNA – Advice and confirmation of amendment to issued undertaking
• ADVA – Advice of amendment to issued undertaking

The above codes when used in amendment must be related to the purpose code defined during issuance of the related Undertaking contract.

Any fields that are changed in an amendment transaction (except the fields on the Amendment/ Amendment Dates panel itself) will be indicated by either red or green color if changed.

• Red color is used to indicate manual modification.
• Green color is used when the field is changed through an incoming electronic message.
• When the cursor is placed over the field (either modified manually or via
  incoming message), the old field content will be displayed as a hint.

When you have made your changes, click on the ( ) Save function to save all the the data entered in the transaction to the database.

Please refer below link for Guarantee Amendment article,

Request for Amendment of Guarantee