In your case, just go back to the main branch, go back to the list of branches - click on the fix1 branch and click "merge with the current" - this will combine fix1 with the main one.
More general explanation:
First of all, make sure you commit your changes before the merge.
Suppose you have a branch named A and a branch named B, and you want to merge B with A, so you need to do the following:
Make sure you are on branch A, you can see it by right-clicking on your project -> Git -> Repository -> branches. The current branch name will appear at the bottom of the list of branches (or at the top of the list marked in yellow from the latest version of Android-studio)
From this list of branches, select the branch that you want to merge with the current branch, and select "Merge", in this case, select branch B and click merge.
If there are no conflicts, you are done. If there are conflicts, then you need to resolve them - Android has a good tool for merging - however, you can easily do it yourself - conflicting files will be painted red - you will see conflict areas with both branches - delete the unwanted code - after resolving all conflicts you need to press CTRL + A to add them to git - and then commit your branch again -> Done.
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