Short answer

Use an absolute cell reference or a named range

Explanation

Instead of E1, which is a relative cell reference, use $E$1 which is an absolute cell reference.

An alternative is to to assign a name to the cell E1, let say, “constant”

In the first case the formula will be

=A2*$E$1

In the second case

=A2*constant 

References

• Spreadsheet – Wikipedia
• Name a range of cells – Google Docs Editors Help