Even if your motors KV are for 4S LiPo (for example 2208 2500KV), you can still put a 6S LiPo on your quad. This is a new feature in Betaflight that auto adjust motor output to compensate for the voltage difference.
Go to PID Tuning tab, and select PID Profile 1.
Under Motor Output Limit, set the Scale Factor to 100% and Cell Count to 4. Press the Save button.
This is the PID Profile for your 4S battery.
Now select PID Profile 2.
Under Motor Output Limit, set the Scale Factor to 66% and Cell Count to 6. Press the Save button.
This is the PID Profile for your 6S battery.
Now when you plug in your 6S battery, Betaflight will automatically scale motor output down to 66%. You can think of it as “Betaflight automatically reducing the motor KV for you”, although it doesn’t really quite work that way.
Note that Betaflight would select the PID profile based on the battery you plug in, so if you make changes to PID or any other settings in the profile, you might want to copy them across profile 1 and 2.
Also note that if you have a 6S setup, you can’t scale up motor output to 150% and hope it would work the same on 4S LiPo. You can only reduce motor output you can’t increase it.