User with erroneous password

This is for a MythTV database.

$ sudo mysql --defaults-file=/etc/mysql/debian.cnf mysql

mysql> SELECT User,authentication_string,Host from user;
+------------------+------------------------------------------------------------------------+-----------+
| User             | authentication_string                                                  | Host      |
+------------------+------------------------------------------------------------------------+-----------+
| mythtv           | *B5BCD029F2268798922CDC55B5253D354B2C0246                              | %         |
| debian-sys-maint | $A$005$F5	%d'zoHMtJEX88t/x1bvIEMnwtseub5Tc7Z02gRpckab8.tZPnvPL5 | localhost |
| mysql.infoschema | $A$005$THISISACOMBINATIONOFINVALIDSALTANDPASSWORDTHATMUSTNEVERBRBEUSED | localhost |
| mysql.session    | $A$005$THISISACOMBINATIONOFINVALIDSALTANDPASSWORDTHATMUSTNEVERBRBEUSED | localhost |
| mysql.sys        | $A$005$THISISACOMBINATIONOFINVALIDSALTANDPASSWORDTHATMUSTNEVERBRBEUSED | localhost |
!ug2Ym[S%/K+ca2n7M06VWGOgOzJFxxKkrg/c7pjNT6Dm3n3FzLXm56 | localhost |
| root             | *2B2E29BFA4C432ED2C7C49E07B175220796B98EE                              | localhost |
+------------------+------------------------------------------------------------------------+-----------+

I want to change the authentication string in the first line with 'mythtv'.
I also want to remove the penultimate line which appears to have no user(?)
This in MySQL version 8.0.
Nothing I do succeeds.
Can someone put me straight?
TIA



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Mint 20.3, kernel 5.4.0-95-generic, Cinnamon 5.2.7
running on an AMD Phenom II X4 Black edition processor with 16GB of DRAM.