One of the biggest advantages of buying or downloading an EA as source code instead of a locked .ex5 is that you can change it. Adjust the strategy, tweak the risk, add a filter. But editing code you did not write can go wrong fast. Here is how to do it without breaking a working robot.
Start with a backup
Before you touch anything, make a copy of the original .mq5 or .mq4. Name it something like MyRobot_original.mq5. If an edit breaks the EA, you can always go back to a known good version. This one habit will save you hours.
Understand before you change
Open the file in MetaEditor and read the top of it first. Most EAs declare their settings as input variables near the top:
input double LotSize = 0.10;input int StopLoss = 50;input int TakeProfit = 100;
These are the safest things to change. They are meant to be adjusted, and changing a default value will not break the logic.
Safe edits for beginners
- Default input values: lot size, stop loss, take profit, magic number.
- Comments and labels: text shown in Print, Comment, or Alert calls.
- Adding a new input: declare it at the top, then use it in the logic.
Edits that need more care
- Entry and exit conditions: changing when the EA trades. Powerful, but easy to get wrong.
- Order handling: how orders are opened, modified, or closed. A mistake here can send bad orders.
- Money management: position sizing formulas. Test these very carefully.
Compile after every change
Make one change, then press F7 to compile. If you get 0 errors, the code is at least valid. If you make ten changes and then compile, and it fails, you will not know which change caused it. Small steps.
Always test on demo first
After a successful compile, run the EA in the Strategy Tester and then on a demo account before it ever touches real money. Compiling clean only means the code is valid, not that the strategy still makes sense. Watch how it behaves for a while.
Keep visible text in English
If you plan to share or resell your edited version, keep every message a trader sees, in Print, Comment, and Alert calls, in clear English. It looks far more professional and reaches a wider audience.
Where to practice
The safest way to learn editing is on code that is meant to be edited. Our free source code library is full of EAs you can open, change, and recompile freely. Once you are comfortable, our premium source codes give you more advanced strategies to build on. And if you want to sell your own version, read our guide on rebranding and reselling MQL EAs.