I'd like to know if I can configure my SAMD21 to run at 8Mhz, is it possible to use 48MHz internal clock and divide it by 6? I need to disable External 32KHz crystal/clock oscillator ?
Yes, it's possible, in fact the SAMD21 already has an on-chip 8MHz oscillator (OSC8M).
The issue though is that the Arduino core code (if you intend to use it?) is designed with the microcontroller running at 48MHz. Switching to another frequency will adversely affect all the timing functions: delay(), millis() and micros(), analogWrite(), etc...
If the purpose is to save power, it might be better to run the CPU at full speed and put it to sleep when not required.
Yes I used Arduino code core, so if I used it samd21 runs at 48MHz after setup?
Yes, after exiting the bootloader entering your sketch, the board switches over to run at 48MHz using it's Digital Frequency Locked Loop (DFLL48M) clock source.
The title says how to increase speed, i’m confused