how to change verification code?

Here is how to request a verification code, though I don’t have an iOS 12.5 device handy to check this:

Get a verification code and sign in with two-factor authentication - Apple Support

Verification codes cannot be re-generated by users.

New verification codes can be requested through various means.

What’s the particular issue or case you’re encountering?

Is this verification code question arising with the iPhone 5s?

Checked for and applied any available updates for your Mac or iPhone or iPad?