These are FCBGA CPUs - soldered into the system board.  They cannot be replaced.

The only upgrade is if there's a system board for your model with a higher-end CPU option - the entire board must be replaced.  This will likely run you well over $300 if it's even possible, and yes - it will void your warranty.

Ok, well I've tweaked my existing setup as far as I can but am still experiencing too much lag for my liking.

What other ways can I boost performance ? Can I buy an external CPU that will boost the processing power. What about a SSD drive- will that only boost program loading times or will it also boost web page loading times. I've noticed I.E is becoming increasingly sluggish-possibly because I've disabled 95% of all add ons and am now finding edge a much more smoother experience ? Am close to admitting defeat and doing a clean boot/factory reset but surely there must be an alternative way...

No, to the external CPU - and while an SSD will boost performance, anything that needs CPU or graphics will still be challenged - this is a weak CPU/GPU, and there's nothing you can really do about that except replace the system with one having a more capable processor.

will swapping the laptop's harddrive for a ssd invalidate the warranty ? Is the reason for my sluggish i.e due to my isp ? am I not getting the responsiveness I seek due to my connection ? How do I test this ?

In general, replacing the drive won't void the warranty -- on most models, it's user-replaceable.  On others, it's not - depends on the system model.

You can test your network bandwidth at speedtest.net (or a similar site).  It's more likely the system's CPU and GPU are holding you back - this is not a high performance system;  that CPU is designed to be a lower-power tablet-style CPU - not a performance notebook CPU.

Could it be the speeds in getting from my isp that are slowing down things. ? I will check when I get home. Should I contact dell to see whether the inspiron 3552 will cope with a 1tb ssd drive. I just use the laptop for basic word office tools, outlook, retro gaming, movies, youtube and music.

Would you recommend overclocking the CPU and if so what steps should be taken ?

That system should be OK for word processing,email and web browsing.  Just about anything else is more demanding than its capabilities - forget gaming, and anything demanding video-wise (youtube OK but anything requiring decoding will swamp it).

You'll find a 1T SSD WILL speed things up but probably cost as much or more than the entire system did initially.

There's little you can do to speed up a system at this end of the market -- it's just a very low-end, limited system.

what about increasing the RAM ? Im going to look around town to see if I can get a cheap SSD drive and have contacted my ISP about optimising my broadband connection.

Even if you can install more than 8G, it's not going to help performance.

The bottom line is that you must realize this is a budget model - there's not much (other than an SSD) you can do with it -- and even that won't help its weak CPU.

not even if I attempt to overclock the cpu ? ( once I learn how to do it ) or do I risk killing my laptop stone cold.

It isn't even possible -- nor will it help.