Do you mean the Samsung Theme Store or do you mean a theme you've downloaded via the Samsung Theme Store @Tyrell1
If you do mean a theme you've downloaded then long press on a blank part of your home screen and choose themes.
Then choose the 3 lines icon in the top Left hand corner followed by My Stuff.
The themes you've downloaded are there for you to choose to apply or delete.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.