how do I upgrade my memory on my iMac 21.5 inch late 2013

See the following footnote from Install memory in an iMac - Apple Support

footnote 3 -Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), iMac (Retina 4K, 21.5-inch, 2017), and iMac (Retina 4K, 21.5-inch, 2019). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorized Service Provider. If you’d like to upgrade the memory in one of these models, an Apple Authorized Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorized Service Provider offers memory upgrade services.

Also, he following well-regarded source of Mac memory rates the skill level needed as "Advanced" and recommends professional installation: https://eshop.macsales.com/shop/memory/iMac/2012_21.5/DDR3L

The Late 2013 iMac's memory is not user upgradable.

Not impossible, but it pretty much be gutted in order to get to RAM modules on the back side of the logic board.

see the Install Video at > https://eshop.macsales.com/shop/memory/iMac/2012_21.5/DDR3L

Between 2009 and 2011, 21.5" iMacs had four user-accessible RAM slots, behind small upgrade hatches.

Once iMacs got really thin (and lost their built-in optical drives), the 21.5" models lost their RAM upgrade hatches and half of their RAM sockets. The other two sockets were sealed inside, where you had to tear apart the Mac to get to them – not an easy job.

There were some subsequent 21.5" models on which the RAM was not merely sealed in, but soldered in (as it is on all 24" M1 and M3 iMacs).

MacTracker shows the "Built-in Memory" for 21.5" Late 2013 iMacs as being 8 GB or 16 GB. If the OP has 8 GB, he or she might want to check that the cause of slowness is lack of memory, and not, say, booting off a 5400 rpm hard drive. If that Mac has just a hard drive (no Fusion Drive or flash storage), getting an external USB 3.0 SSD and using it as a startup drive might be an easier and more promising upgrade.

The Activity Monitor's Memory Pressure graph can help in determining if this is the case.