I found this video for you, I havn't tried it but I hope it works.
FOR THE 1ST OPTION
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If your on windows you first click the windows button on your keyboard or at the bottom left or wherever on your toolbar, type in the Search Programs And Files %appdata%
click on the .technic option and then click modpacks then select the modpack you want then change what the video says in the options.txt
FOR THE 2ND OPTION
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Click the windows button on your keyboard or bottom left on your toolbar, Below your computer name 4 items below and right click the computer option and click proporties
and by the Installed Memory (RAM) and just do the rest of the video.
Sorry If I didn't help that much.