Elastic to Fixed IPv4

Hello.

I have an instance with an Elastic IP and I need a fixed IPv4 address, is there a quick way to make this change?

If it is an EC2 with an Elastic IP address attached, it should already be a fixed IPv4 address.
If ElasticIP address is not attached to EC2, you can set it by following the steps described in the document below.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html#using-instance-addressing-eips-allocating