There is no shame in correcting your mistakes (such as off-beat or off-key notes) after recording a Soundtrap instrument.
It's very common among professional musicians and you should do it too. Quantizing is the process of automatically adjusting the timing of your MIDI notes so that they're perfectly in sync with the rest of the music.
Note: Currently, quantizing is only applicable to MIDI/Soundtrap instrument regions
First, open the piano roll. This can be done either by double clicking the recorded region or hovering over the region and clicking "Piano roll" (1).
Next, select all the MIDI notes you want to quantize (pro tip: press Ctrl/Cmd+A to select all). Then right-click anywhere to open the Piano roll menu, and select "Quantize" with the appropriate beat division (2).
If you've played for example a fast hi-hat or synth arpeggio, you might want to go with one of the higher divisions (1/32) but for the most part 1/8 or 1/16 will do the job!
(1) Open piano roll
(2) Quantize notes