I think that is no way to replace a certain line in your text file...
Right, probably that have a way (always have), but maybe too hard... ;]
Using the commands vb5prgrmr told you (Open / Line Input / Print / Close) you will be able to:
1. Open your file, for READING;
2. Put EACH line you read in a invisible multiline textbox or in a string variable;
3. Close your file;
4. Open it again, for WRITING;
5. Begin with the first lines... then, when you reach the line "LineUser=..." (you can use InStr function to test if each line matches, or counting lines to the nth. line), simply write the new value you want (like Print #1, "LineUser=" & Text1.Text );
6. Write the other lines without modification (remember you stored these lines in textbox or variable);
7. Close your file.
If you need, try googling "OPEN STATEMENT VB6" or something else to get more help on how to manipulate files. You'll find a lot of examples... The MSDN is a good start, also.