Can I change my handle after signing up?

Yes, in Account Details. Your handle is checked for availability as you type and must be three to thirty characters using only lowercase letters, numbers, dots and underscores.

One thing to know before you do it: your handle is your public address. Change it and wegrownepal.com/u/your-old-handle stops resolving, so any link to your profile that you have already shared will break. The notes themselves are unaffected — they live at their own addresses.