Hi,
That really depends on what specific changes you’ve made (and what version of the theme you’d be updating from).
As a general rule it’s a bad idea to make changes directly to themes or plugins installed from the repository (or third party sources). For themes you can usually use the Additional CSS field for just CSS tweaks or implement a child theme for more complicated customizations.
Since updating now would actually require to transfer these changes to the new theme version (the opposite of implementing the theme update changes would most likely require more work), I suggest you take the opportunity to also transfer the tweaks out of the theme’s files.