I have a QNAP TS-453DB NAS box, which pretty much just gets used for mass storage. Recently I started doing more video work, and the storage just wasn't enough anymore. I had 4 x 1 TB drives in RAID 10 for 2 TB of storage (yes I am a stinge). I was getting a lot of email complaints from the QNAP about low memory as well (I bought the 4 GB model), which rendered it unable to complete anti-virus scans.

So I upgraded the drives to 4 x 6 TB Seagate Iron Wolf for 12 TB of storage, which was straightforward, and upgraded the RAM to 8 GB by swapping in a new memory module. There are a few annoying details that are worth documenting so you don't have to do it yourself:

Upgrading the RAID capacity

This is fairly straightforward, but you must backup your data to an external drive or two first, in case this goes wrong, because this process involves degrading the RAID array multiple times. Obviously, you should also have a set of new larger hard drives on hand, preferably identical model and capacity.

I recommend you write the drive number on each hard disk label, so that if you ever take them all out, you know which bay they need to go back into!

1. Start by going to Control panel => Storage => Storage/Snapshots and right clicking on the Storage Pool (not the volume), and selecting 'Manage' from the context menu.
2. A dialogue box opens displaying the RAID group information. Click on the 'Manage' drop down box and select 'Replace Disks One by One'.
3. Click on a hard drive to highlight it (you may as well do them in order, eg. start with HDD 1) and click the 'Change' button. It will then tell you to remove that drive. AFAIK, QNAP numbers the drives from left to right as you look at the front of the unit.
4. Pull the old drive out, swap the new larger drive into the tray, and re-insert it. The QNAP will beep and complain multiple times, don't worry. 
5. Now you have to wait. The RAID is now degraded and the QNAP will rebuild it incoporating the new drive. This took about 90 minutes for my 6 TB drives, if you are using larger ones it will take longer.
6. When it is is complete, repeat the process with the next drive, until you have swapped out the full set.

The RAID is now upgraded, but you also need to expand your data volume(s) to make use of the additional space.

1. Go back to Control panel => Storage => Storage/Snapshots, right click on a data volume and click 'Manage'. 
2. Click on the 'Actions' button and select 'Resize Volume'.
3. Adjust the capacity and select 'Apply'.
4. Repeat for other volumes as required.

Now you have to wait again, while the QNAP processes your change. It was another lengthy process and was still going when I left for work the next morning, but was done by the time I got home.

Upgrading the RAM

The main issue with upgrading the RAM is making sure that you pick a compatible module, and if you're going to install two modules then it is best to make sure they are identical. According to QNAP the TS-453D supports up to 8 GB, but online people have reported successfully installing 16 GB. Frankly, there's no point installing more than 8 GB.

After some research online (check the QNAP forums) I bought a single Crucial 8 GB DDR4-2400 SODIMM, model CT8G4SFS824A. It works fine.

1. Power down your QNAP, unplug it, press the power button to dissipate any residual charge. Then take the drive bays out (you did write drive numbers on the labels, right?). You do not need to take the case off.
2. Once the drive bays are you you can see the memory module, which is held in place with clips on the sides. Gently push the ends of the clips outwards so they slide off the edge and the module will pop up (note the angle).
3. Swap in the new one (insert on the same angle), when it fully in, gently push it back to lay it down, and push the side clips back on to the edges. There are YouTube videos if you want to see how it's done.

There's nothing more to, just plug in and power on and it should just work.