From the Lobby click on the bottom right gear as shown below:

After you click on the Gear/Game Menu you can then click on Settings to go to the Settings page:

Changing Toggle Aim For MNK

If you are on Mouse and Keyboard you need to go to the Mouse and Keyboard tab, scroll down and look for Aim Down Sight (Hold) then set a button or overwrite it with right mouse click. Something you can only do on PC is to set the Toggle and Hold buttons to two separate keys. This can’t be done on Controller.

Changing Toggle Aim For Controller

For this one you need to go the Controller tab. Then look for Aim Button then set it as Toggle or Hold:

We have other articles on Apex Legends, if you are getting started we would recommend you to check: How To Get Better At Apex Legends