Say something silly, Laugh 'til it hurts, Take a risk, Sing out loud, Rock the boat, Shake things up, Flirt with disaster, Buy something frivolous, Color outside the lines, Cause a scene, Order dessert, Make waves, Get carried away, Have a great day!
I assume your using the Twitch launcher (or ATLauncher) and have downloaded RLCraft not put it into the Vanilla launcher right?
Otherwise technically a modpack should work and not need Forge to be updated but if it's through Twitch you update Forge by going to the profile options of the modpack and change the modloader version. (Picture 3)
If you want to update Forge for the Vanilla/Mojang launcher just go to the Forge website, download installer for the version of Forge your after and then click the installer, install client, let it get what it needs, once it's done launch the Vanilla launcher, go to installations then it should have a new or old Forge install there (Picture 2), and to change it to the updated just edit that Forge profile to the Forge you just installed. (Picture 3)
If you need any more info or help just let me know.
Vanilla Launcher Profiles
Editing a version Vanilla or Modded
Profile settings to update Forge, change RAM usage and more