If you want to give your aging machine an even bigger boost, I'd strongly suggest opting for a new SSD (flash) drive instead. With your system and apps on it, you can get work done much faster as everything will go from sluggish to snappy.
Either way you go the process is the same and, aside from one long data transfer you can sleep through, it only takes a few minutes.
Step 1: What you need
- new 2.5" HDD or SSD drive
- external 2.5" USB enclosure ($10)
- #00 Philips screwdriver
- T6 screwdriver
And 2 software options
- Carbon Copy Cloner - free for 30 days, $40 after
- SuperDuper! - free for basic use (all we need here) and $28 for full services
I've used both applications for this and both worked equally well for the basic application of hard drive cloning.