Note that your question is about level reduction, rather than modulus switching. Modulus switching in CKKS is rescaling. In BGV/BFV, modulus switching scales down the noise while in CKKS is scales down both the message and noise.
In the FIXEDMANUAL mode OpenFHE often automatically calls level reduction. If you want to do it manually, I suggest reading How can i do levelreduce explicitly while using flexibleauto mode Basically you can call the LevelReduceInPlace method of CryptoContext for this.