Hi Adel,
the detailed information is available in the following KB article: https://portal.nutanix.com/kb/5505
In short, you can switch between LTS and STS, but only to a newer version. Right now (as of 4th of January 2023), the latest version is 6.5.1.8 LTS. There will be soon the version 6.6 STS. If you upgrade to 6.6 STS, you will not be able to go back to 6.5.x LTS versions, because that would be a downgrade and downgrades are not supported. You will only be able to go to LTS when the next LTS is released (most likely it will be 6.10.x, but not yet confirmed).
If you are on an earlier STS version now, for example, 6.1 STS, you can simply upgrade to the newer LTS, which is 6.5.1.8 for example.