How do I replace characters in the middle of a string?

[Adam Yik]

How do I replace a character/a group of characters in each line? For example:
https://www.example.com/images/asdfuisdhfaskehtiohgs_50.png
https://www.example.com/images/sdfjksaldfjalsjdkdddd_20.jpg
https://www.example.com/images/shgrfjkcviofreavcdfsd_600.jpg
https://www.example.com/images/disolkhrewiousdfuoidd_400.png

I want it replace it with:
https://www.example.com/images/asdfuisdhfaskehtiohgs_1280.png
https://www.example.com/images/sdfjksaldfjalsjdkdddd_1280.jpg
https://www.example.com/images/shgrfjkcviofreavcdfsd_1280.jpg
https://www.example.com/images/disolkhrewiousdfuoidd_1280.png

I want to replace any amount of digits without replacing “_” and anything before it and I do not want to overwrite anything after the dot “.”. I want to replace something in between it (the numbers).

And I do not know how to use the regular expressions for replacing something that is next to a character I do not want to replace.

[Adam Yik]

Solved, realized how the brackets work, the solution is this: [_].*?[.]. Sorry!