I am getting my battery replaced today will I lose anything on my phone by doing this?

Possibly. Apple may need to wipe the iPhone during battery replacement. But this should normally not be an issue.

You should have a backup made of all the data either on iCloud or on a computer.

iCloud can be set to automatically backup when the iPhone is connected to Wi-Fi and power and the screen is turned off.

You can also initiate a backup at any time from iCloud Settings on the iPhone or through iTune or Finder on a computer (Mac or PC).

If you can still access the iPhone, make a backup before taking it in.

click here ➡️ How to back up your iPhone, iPad, and iPod touch - Apple Support

To expand on Phil0124's post, you should be backing up your phone regularly. You can set it to back up nightly to iCloud. This is the ideal. If you wait until you think you might need a backup, it's probably too late. Turn on iCloud back up then, every night, put your phone on the charger and lock the screen. You will then never lose more than a day's worth of data if your phone is lost, stolen or damaged.