To update the weight of a patients there are two general paths. If there is an encounter open for the patient, you will have access to change the weight right from the patient dashboard. If however this is outside of an encounter, your best option is a weight note.
For this you would open the patient context menu on the patient dashboard and select "Enter Note":
Now in the note editor you change the type to "Weight Note":
Next you just add the weight to the weight box and then click "Create Note"
The weight will now be update for the patient and there will be an entry recording the change as a note.