If you're looking to manually set your Gas Fees, to ensure that your transaction gets confirmed fast and executed successfully, you can do so in the MetaMask UI by enabling advanced gas control in Settings > Advanced:
Click on the Account icon:
Then, click on "Settings":
Then select "Advanced":
In "Advanced" please scroll down:
Then, enable Advanced gas controls:
The result should be the following:
In addition, please keep in mind that:
The Gas Price is what motivates miners to pick-up your transaction and include it into the next block that gets processed, so your transaction is confirmed on the network. The more Gas you use, the higher priority your transaction will have over other transactions.
The Gas Limit is used to calculate the number of steps that the Ethereum Virtual Machine can take to process your transaction. If it's too low, the Gas costs will get consumed before reaching the end of your transaction's instructions, leading to a transaction not being executed due to running out of gas.