You can easily adjust stock within your inventory and then have it sync to all your sales channels immediately.
To do this, click on the stock level to bring up a dialog where you can either make an Offset or Total adjustment. Doing an Offset is useful for stock takes (i.e. receiving new stock) while Total is useful for fixing stock levels.
For example, if the current stock level was 5:
Inputting an Offset of 3 will adjust stock to 8
Inputting an Offset of -5 will adjust stock to 0
Inputting a Total of 10 will adjust stock to 10