How to migrate my "m1.small" reserved instances to "t3.small"?

Is your reserved instance Standard or Convertible? If it's convertible - you can use the exchange feature to change the instance family & generation, and select any instance family within the same region. However, if it's Standard, there's no way to change the instance family or generation, that's just the way Standard RIs are designed.

Generally, RIs cannot be cancelled, or you cannot change the family or generation if it's Standard RI, however I still recommend opening a support case (billing support case) from the account where you purchased your RI. You can provide the region of RI and its lease ID (alphanumeric reservation ID) to support team, and ask them how they can help with replacement, considering that Classic platform is deprecated and you have to modernize your instance to run it in VPC.

As a general rule, it is best practice to modernize your infrastructure and use most recent generation instances before you reserve. Newest generation instances always offer better performance for better cost.