Do you mean change the wavetable for an oscillator or change a single waveform within a wavetable?
Selecting a different wavetable for an oscillator is easy - either click on the middle tab directly beneath the wavetable image you want to change and select from the dropdown, or click "tool" above the wavetable image and then on "load wavetable (N*2048 samples, icarus, serum, falcon) and select the wavetable via the file browser that opens. That should just replace the wavetable for the chosen oscillator, leaving all other settings intact.
To change a single waveform in a wavetable, click on "edit" above the wavetable, then click on the magnifying glass top right until you can make out individual waveforms, and scroll across so you can see the one you want to edit by using the scrollbar directly beneath the waveforms display, click on the waveform you want to edit, then you can modify the wavetable by drawing directly onto the big waveform display using the tools bottom right, or edit via "spectrum" and "phase". You can also click on "file" and then "load single cycle waveform to selection" to replace the single waveform with one from disk.
