I want to update my desktop, can I bring in my old desktop and get all my photos and iTunes transferred?

Sorry, how do you mean by update? As in to update the Operating System of the iMac, or as in to physically upgrade the iMac to a newer/more recent version? I expect its the latter. I'd of thought both your photos and music libraries will be synced into your iCloud account currently, which if so when you enter your Apple ID/Password into your new updated iMac Desktop, providing you've backed up your content it should simply be a case of re-syncing your items from your iCloud account onto the desktop from there on wards.

When I bought my Mac, yes, I took my old desk top machine (a Dell PC) to the Apple store and they did the transfer there free of charge. Don't know if they are still doing that. It's been over 5 years. However, it is easily done yourself. You may want to check out these links.

https://support.apple.com/en-us/HT204350

https://discussions.apple.com/thread/7446277

BTW, you are not talking to Apple employees on this forum. We are all users like yourself.