If you would like to change the interest on a specific loan within an interest period. go to the transaction tab and click on the icon for fixing interest (A in the picture below).
You then get a popup where you fill in the date from when the new interest should start . Fill in the new base interest and/or the new spread and click OK
Treasury Systems will then change the interest from that date following. If the change is within the interest period a new transaction will be created with the same payment date as the original interest payment