@donko Can you write what all you tried ? What were the errors you faced ?
Switching to mpm_event is fairly straightforward, you do a2dismod phpxx and then do
a2dismod mpm_prefork
a2enmod mpm_event
but please be warned, you cannot use mod_php based templates after this. HestiaCP does not support MPM_event out of the box
As I said before, better if you try this on a test server and just post if any problems so we can help