You can be 100% sure you’re the only one in control of defining the bet. There is no way the bet can change automatically.

When you enter the slot, there is a default bet which is the average one for a certain slot.

The bet depends on your status and it can always be changed from the minimum to the maximum one. It is always up to you. Moreover, please note that the higher your bet is, the greater is your chance to win big.