MultiMC - alternative Minecraft launcher
MultiMC is a free, open source launcher for Minecraft. It allows you to have multiple, cleanly separated instances of Minecraft (each with their own mods, texture packs, saves, etc) and helps you manage them and their associated options with a simple and powerful interface.
Download here: https://multimc.org/#Download
After installing, you need to change memory settings and add profile:
How to add a modpack