Worth to upgrade late 2015 mac to ssd

Well, if you have the standard configuration, your hard drive is a snail speed 5400 rpm spinning drive. Even a 7200 rpm spinning drive would be faster and an SSD much faster.

Professional installation is recommended according to this site which has excellent install videos (as well as hard drives, RAM, etc):

https://eshop.macsales.com/installvideos/imac_21inch_late_2013_hdd/iMac16-2/

So you can check prices and then visit an authorized service provider and ask them how much it would be and then make your own decision. An SSD will help, but if you cannot add more RAM, that will be a roadblock with some apps (which are resource hungry).

A much easier and cheaper way to upgrade to an SSD is to

purchase either an external enclosure and install an SSD in it

or one of the myriad ready to go SSDs currently on the market.

Then, move current installation on to it and use it as

a boot drive.

There are two ways to accomplish this. The easiest would be to create

a clone of your internal drive on the external or install your current version

of macOS on the new external drive and use Migration Assistant to move

all your internal data and apps to it. Once either of these are complete,

simply open System Preferences->Startup Disk, set the external as a

boot drive, and reboot.

Total cost, the price of the external SSD drive and if doing the clone approach,

the cost of the cloning app (some are free) and some time. Much less than the

$400 quoted ($150-200 and perhaps cheaper with a bit of shopping) and a

very significant speed boost. The big plus, your iMac remains sealed and untouched.

FWIW, I did this for several years when I first got my late 2013 27" iMac which

had an HDD without any issue.

You say you do basic photo and video editing. Please know that these "basic'" functions are RAM intensive. Therefore, when you decide to get a new computer I recommend that you load up on RAM, Get one with an SSD drive and get one which allows the user to upgrade the RAM by installing or changing RAM cards.

If it was mine, I'd just replace it with a new 21.5" with a minimum of 16GB of RAM and the largest SSD you can afford. That would be a ROCKET compared to what you own.