First, you need to find where your Tomcat's keystore file is. This is usually configured via a
<Connector> element in the Tomcat config, e.g.:
Since you mentioned you already have an existing certificate that's about to expire, it suggests that you already do have a keystore configured like this.
So then, we need to import the certificate/key you have, into that keystore file. To do this, you first create a PKCS#12 file from your
something.key files, and then import that PKCS#12 file into your keystore file (e.g.
/path/to/keystore.jks) using the
keytool command; see https://stackoverflow.com/a/11954816/4072843 for examples of how to do this. (Note that your
something.csr file is not needed for this; the
.csr extension suggests that that file is the Certificate Signing Request file, which is how you requested that certificate from your CA; a CSR file is not, in itself, a full certificate.)