Versioning in REST API?

Question

PHP

There is an implementation of some rest api.
The oldest version, v1, has the path site/api by default. The newer one has site/v2/api.
Now in v2 we need to make incompatible changes to one method. It is not reasonable to change to v3 because of this, since 99% of the functionality remains the same.
It was decided to introduce semantic versioning according to the semver.org specification for the second implementation of the REST API. Those. api v2 will have its own versioning.
For accounting, I consider the library https://github.com/nikolaposa/version
The question arose, how best to take into account the version in the code?
Those. it's stupid to write a condition in the code, if the major version of the requested API is less than 2, then I execute such code, otherwise another one?
Are there any good practices?