Firstly you should be using parameterised queries - I'll leave you to look that up.
Secondly, you must separate the fields your are using in your sql statement with commas - as your query currently stands it should be
UPDATE Produktet SET ID=" + txtID.Text + ", Pershkrimi= '" + txtPershkrimi.Text + "',Njesia='" + txtNjesia.Text + ",'Sasia=" + txtSasia.Text + ",Cmimi=" + txtCmimi.Text + "";
Edit - here's a link to learn about parameterized queries
http://www.dotnetperls.com/sqlparameter[
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