Here is a code snippet to change the mdf and ldf file path of an existing database to instance default path in SQL Server.
If you want to move to fixed path, then you can change the script as required.

Assumptions:
1. The script is mainly written for SQL Server 2016, for other version, please change as required.
2. The script will not execute or change the path automatically. It just prints the statements to execute. The user can validate the scripts and do the action as required. This is to prevent any unexpected events.

--Print 'Collect Default Data & Log information'
declare @DefLOG nvarchar(512)
declare @DefDATA nvarchar(512)

--Prepare the SQL statements for remapping
if (Cast(SERVERPROPERTY('Productmajorversion') as varchar(2))='13')
Begin

Set @DefLog = Cast( Serverproperty('InstanceDefaultLogPath') as varchar(512))
Set @DefDATA = Cast( Serverproperty('InstanceDefaultDataPath') as varchar(512))

--Print 'Move all datafiles'
declare cur_move cursor for
select --DB_NAME(dbid),name,filename, reverse(SUBSTRING(reverse(filename),1,CHARINDEX('\',reverse(filename))-1)),
'ALTER DATABASE ['+DB_NAME(dbid)+'] MODIFY FILE ( NAME = '''+name+''' , FILENAME = '''+@DefDATA+reverse(SUBSTRING(reverse(filename),1,CHARINDEX('\',reverse(filename))-1))+''' )'
from sys.sysaltfiles where dbid <32000
and groupid=1 and dbid>5 and charindex(@DefDATA,filename)=0
union all
select --DB_NAME(dbid),name,filename, reverse(SUBSTRING(reverse(filename),1,CHARINDEX('\',reverse(filename))-1)),
'ALTER DATABASE ['+DB_NAME(dbid)+'] MODIFY FILE ( NAME = '''+name+''' , FILENAME = '''+@DefLOG+reverse(SUBSTRING(reverse(filename),1,CHARINDEX('\',reverse(filename))-1))+''' )'
from sys.sysaltfiles where dbid <32000 and charindex(@DefDATA,filename)=0
and groupid=0 and dbid>5

declare @move varchar(max)
open cur_move
fetch next from cur_move into @move

while @@fetch_status=0
begin

print @move
--Exec (@move)

fetch next from cur_move into @move
end
close cur_move
deallocate cur_move

End
Else
Begin
Select 'This script supports SQL Server 2016 or later! For older version, you need to below to identify the default path, otherwise all are same.'
exec master.dbo.xp_instance_regread N'HKEY_LOCAL_MACHINE', N'Software\Microsoft\MSSQLServer\MSSQLServer', N'DefaultLog', @DefLOG OUTPUT
exec master.dbo.xp_instance_regread N'HKEY_LOCAL_MACHINE', N'Software\Microsoft\MSSQLServer\MSSQLServer', N'DefaultData', @DefDATA OUTPUT
return;
End