hi expert, how to modify pda field in my program

i defined a struct:

#[account]pub struct AcctEntity {    owner_id: Vec<u8>,    big_brothr_acct_addr: String,    aaa_addr: String,     question_nos: String,    bump: u8,}

after i create it . I want to modify the field 'aaa_addr', how to write the code in anchor?use assignment is invalid....