How to do it

Preperations 
 You need to be in a rescue system or have the disk unmounted. That is non negotiable! Also make a snapshot/backup of you system! 
 
 Resizing 
 
 e2fsck -f /dev/sdXY 
 resize2fs /dev/sdXY XG 
 fdisk /dev/sdX 
 
 In fdisk you need to: 
 
 Delete the old partition (d) 
 Recreate it (n)
 
 Use +XG to resize it to the GB amount you choose when using resize2fs 
 
 
 Write it (w) 
 
 Now you should have a working partition that is resized to the amount you wanted.