Well I have no clue on what to tell you to upgrade to because you haven't described to me what you'd like to do with the CPU. Though if it were me... Personally. I'd go with something similar to this.
PCPartPicker part list:
https://pcpartpicker.com/list/FsZHhy
Price breakdown by merchant: https://pcpartpicker.com/list/FsZHhy/by_merchant/
CPU: Intel - Core i7-8700K 3.7GHz 6-Core Processor ($347.79 @ OutletPC)
CPU Cooler: Cooler Master - Hyper 212 LED 66.3 CFM Rifle Bearing CPU Cooler ($24.89 @ OutletPC)
Motherboard: Gigabyte - Z370P D3 ATX LGA1151 Motherboard ($99.89 @ OutletPC)
Memory: G.Skill - Ripjaws V Series 8GB (2 x 4GB) DDR4-3200 Memory ($103.99 @ Newegg)
Total: $576.56
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-03-31 22:31 EDT-0400
It only has 8GB of RAM though but rather than cut down on CPU I'd choose to upgrade the RAM later.