Here's some general advice - we can get more specific if you can get specific about your computing needs, such as where does your current system not measure up, what tasks are slow now, and what additional capabilites do you want the computer to have.
I advise against upgrading your current system to its fastest processor, for the following reasons.
1. This is a 2011 motherboard platform. Technology has improved, and the older platform cannot equal current processing, graphics, or memory technologies, even if it is maxed out.
2. Upgrading to the fastest processor for a platform is usually not cost efficient, especially after those processors are no longer available at retail. This is because pricing ramps up due to market demand from others in your scenario, and a slowly dwindling supply.
In your case, the fastest processor for the platform is A8-3870K. This would still be 4 cores, with a fair increase in CPU frequency from 2.1 GHz to 3.0 GHz, plus a slight boost in graphics capability, and no change in memory capability (both processors should run DDR3 at 933 MHz, if the motherboard supports it). This processor costs about $35 second-hand on eBay. Honestly, that is less money than I thought it would be. Therefore, one option would be to ignore my reasons above, get the A8-3870K for $35, and see if it gives you a significant performance boost. However, I believe it will be only a minor improvement.
OK, recommended replacement. If your workload is similar to 2011, then you could get a similarly configured current generation system such as Ryzen 5 Radeon 2400G. The Ryzen architecture is much faster, this would be 4 cores / 8 threads at 3.6 GHz (boost to 3.9 GHz), much better Vega-based APU graphics, and support for DDR4 at 1466 MHz. The new motherboard would likely have current generation ports like usb 3.1, etc.
If your workload has increeased significantly, then you might consider more processor cores and a discrete graphics card. Ryzen 5 2600X has 6 cores / 12 threads at 3.6 GHz (boost to 4.25 GHz) and the same DDR4 memory support. Add a Radeon RX580 or whatever discrete card suits your needs.