Can't edit some random place in code without deleting everything that comes next

this is really stupid question but I just downloaded Visual Studio and it's really driving me crazy. Lets say I made a mistake somewhere in code and want to edit it, when I click on that part there is grey thing highlighting the number or letter I am currently on but when i try to add something it is just deleting everything on its path, so if I want to add something i have to retype everything that comes next, here is screenshot because I can't explain it very well..


Lets say I wanted to separate fi from le but instead it removed my i and if I keep pressing space it will also remove l and e.

Thanks in advance