RTFM!
Kidding 
To replace a sample, scroll through the sample slots on the SRC page, highlight the slot you want to replace, and press FUNC+Yes
If you are affraid to mess things up: Save your project as a new copy and a version number before making big changes so you can always revert back to the previous save, or copy over patterns etc.
General tips on sample management: Choose your samples wisely before uploading them to the +drive. Then start creating grooves and when you need a sample, load it into the project. In stead of just loading a bunch of samples into a project. This way you will end up with 1 project containing great samples that you re-use a lot, because DT can mangle the sample with LFO’s and filters etc. to create a lot of different sounds form one sample.
You can also purge samples. Go to sample browser in settings, press left, select view RAM, then right arrow and select purge samples. It will remove any unused samples.
Additionally, check out Elk Herd for sample management. You can move samples around within the project’s sample slots and import patterns from one project to another including all the used samples etc.
I’ve created a video showing elk herd in detail:
Cheers and welcome to this lovely forum 