I'd love to explain in this post how to allocate more RAM, but it's best to have a visual guide for this. Check out this video:
Be aware that he does have numbers in the description for whatever amount of RAM you want to allocate to Minecraft.
Also, look for an option called "Advanced Settings" near the top of the screen in the Launch Options, just below the Minecraft banner. Make sure it's enabled or you won't be able to change the JVM arguments to allocate more RAM. This youtuber failed to mention that.