Follow the below path to change the installment amount for existing fee plan and also you can add the new installment without modifying the existing installments.

Method 1: Admin Level Access

Settings --> Finance --> Create Fee Plan --> Select the Academic year, Type of Fee Plan and click on Get --> Manage Fee Plans --> Click on "View Settings" against the fee type --> Edit Plan Amount

Method 2: Branch Level Access

Finance --> Fee Management --> Manage Fee Plans --> Select the Academic year, Type of Fee Plan and click on Get --> Manage Fee Plans --> Click on "View Settings" against the fee type --> Edit Plan Amount