Minecraft Launcher for Vanilla

This will adjust the ram per installation. The Default allocated amount is 2GB

1. In the Minecraft Launcher in the top middle click on Installations.
2. Click on the installation you want to adjust to edit.
3. In the editing menu click on more options.
4. Under JVM ARGUMENTS you'll want to change the 2 in-Xmx2G to the amount of RAM you want to allocate, in gigabytes.

Curseforge

This will adjust all ram for your CurseForge modpacks.

1. Go to your settings by clicking on the cog wheel down in the bottom left of the client.
2. Under Game Specific click on Minecraft.
3. Scroll down till you see the Java Settings and adjust the allocated memory.

Technic

This will adjust all the ram for all your Technic modpacks.

1. Go to your settings by clicking on Launcher Options in the top right of the client.
2. Click on the Java Settings tab.
3. Under memory you can adjust allocated memory.

Feed The Beast (FTB)

1. In the Library click the profile or modpack you want to increase the ram on.
2. In the modpack information click on Settings.
3. Under the Java Runtime you should see Instance Memory to adjust your allocated memory.

ATLancher

This will adjust the ram in all your ATLauncher modpacks

1. Go to your settings by clicking on Settings from the Menu in the left hand side of the client.
2. Click on the Java/Minecraft tab.
3. Under Maximum Memory/Ram adjust your allocated memory.

Prism Launcher/MultiMC

1. Go to your settings by clicking on Settings from the top menu.
2. Click on the Java tab.
3. Under Memory adjust Maximum memory allocated .