G'afternoon xlllu and welcome to the forums!

when your main dies, do the following:

1. hit the 'suspend' hotkey (not set by default... check under setup/override settings)
2. hit the PiP swap hotkey for the desired alt
(alt should now be in place of the main... and the dead main is in the alts place)
3. click on the newly arrived alt window (this is to just make sure you have input in that window)
4. hit your 'set focus to me' button for the alt you just pulled in (some people make this the same as the PiP hotkey)
5. seek revenge

let me know if that helps