How can I modify the instance name for an ec2 instance

This might help...

AmazonEC2 ec2;    
AWSCredentials credentials;
String accKey = "your access key";
String secKey = "your secret key";    

credentials = new BasicAWSCredentials(accKey, secKey);
ec2 = new AmazonEC2Client(credentials);

String instanceId = "Your Instance ID";
List<Tag> tags = new ArrayList<Tag>();

Tag t = new Tag();
t.setKey("Name");
t.setValue("my server!");
tags.add(t);

Tag t = new Tag();
t.setKey("owner");
t.setValue("me");
tags.add(t);

CreateTagsRequest ctr = new CreateTagsRequest();
ctr.setTags(tags);
ctr.withResources(instanceId);
ec2.createTags(ctr);

kind of quick and dirty, but you get the idea.

You can do it through AWS console UI:

In 2021 this can be done from the AWS console, by going to EC2 > Instances, clicking the instance ID, then in the bottom panel clicking on the Tags tab, and clicking Manage tags. From there, you can simply change the value of the Name tag then click Save to apply.