Open the Minecraft Launcher
Go to Installations
Select the profile you want to change
Click More Options
Find the JVM Arguments field
Look for the value after `-Xmx`
Change that number to the amount of RAM you want in megabytes
Example: `-Xmx4G` for 4 GB of RAM
Save the profile
Make sure not to assign too much RAM for your system
On the launcher, click Play
For the Microsoft Store / Bedrock version, open the game settings if available and adjust memory-related options through system performance settings
Close unnecessary programs before launching Minecraft
Use a 64-bit version of Java for Java Edition
Make sure your computer has enough free RAM before increasing the allocation