Check out this previous post, is it possible for you to try a different browser?
You can also try using the CLI to modify the instance class.
Can't modify RDS instance classAccepted Answerasked 5 months ago
cannot modify RDS on govCloudasked 9 days ago
Cannot modify RDS instanceasked 2 months ago
RDS Database Instance does not show up in dropdown when creating RDS Proxyasked 6 months ago
rds instance stuck in status stoppingasked 5 days ago
RDS modify instance not workingasked a month ago
Cannot modify the instance class because there are not enough availabilityAccepted Answerasked 3 years ago
DB Instance class droprown is empty when attempting to modify existing RDS Databaseasked 2 months ago
Cannot modify Postgres RDS instance class in us-east-1easked 8 months ago
RDS MYSQL 8.0 parameter group value is not updatingasked 8 months ago